[SPARK-32283][CORE] Kryo should support multiple user registrators

### What changes were proposed in this pull request?
`spark.kryo.registrator` in 3.0 has a regression problem. From [SPARK-12080](https://issues.apache.org/jira/browse/SPARK-12080), it supports multiple user registrators by
```scala
private val userRegistrators = conf.get("spark.kryo.registrator", "")
    .split(',').map(_.trim)
    .filter(!_.isEmpty)
```
But it donsn't work in 3.0. Fix it by `toSequence` in `Kryo.scala`

### Why are the changes needed?
In previous Spark version (2.x), it supported multiple user registrators by
```scala
private val userRegistrators = conf.get("spark.kryo.registrator", "")
    .split(',').map(_.trim)
    .filter(!_.isEmpty)
```
But it doesn't work in 3.0. It's should be a regression.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Existed unit tests.

Closes #29123 from LantaoJin/SPARK-32283.

Authored-by: LantaoJin <jinlantao@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

core/src/main/scala/org/apache/spark/internal/config/Kryo.scala

@@ -29,7 +29,8 @@ private[spark] object Kryo {
   val KRYO_USER_REGISTRATORS = ConfigBuilder("spark.kryo.registrator")
     .version("0.5.0")
     .stringConf
-    .createOptional
+    .toSequence
+    .createWithDefault(Nil)
 
   val KRYO_CLASSES_TO_REGISTER = ConfigBuilder("spark.kryo.classesToRegister")
     .version("1.2.0")

core/src/test/scala/org/apache/spark/SparkConfSuite.scala

@@ -221,7 +221,7 @@ class SparkConfSuite extends SparkFunSuite with LocalSparkContext with ResetSyst
     conf.registerKryoClasses(Array(classOf[Class1]))
     assert(conf.get(KRYO_CLASSES_TO_REGISTER).toSet === Seq(classOf[Class1].getName).toSet)
 
-    conf.set(KRYO_USER_REGISTRATORS, classOf[CustomRegistrator].getName)
+    conf.set(KRYO_USER_REGISTRATORS, Seq(classOf[CustomRegistrator].getName))
 
     // Kryo doesn't expose a way to discover registered classes, but at least make sure this doesn't
     // blow up.

core/src/test/scala/org/apache/spark/serializer/KryoBenchmark.scala

@@ -125,7 +125,7 @@ object KryoBenchmark extends BenchmarkBase {
   def createSerializer(useUnsafe: Boolean): SerializerInstance = {
     val conf = new SparkConf()
     conf.set(SERIALIZER, "org.apache.spark.serializer.KryoSerializer")
-    conf.set(KRYO_USER_REGISTRATORS, classOf[MyRegistrator].getName)
+    conf.set(KRYO_USER_REGISTRATORS, Seq(classOf[MyRegistrator].getName))
     conf.set(KRYO_USE_UNSAFE, useUnsafe)
 
     new KryoSerializer(conf).newInstance()

core/src/test/scala/org/apache/spark/serializer/KryoSerializerBenchmark.scala

@@ -76,7 +76,7 @@ object KryoSerializerBenchmark extends BenchmarkBase {
     conf.set(EXECUTOR_EXTRA_JAVA_OPTIONS,
       "-XX:+UseParallelGC -XX:-UseDynamicNumberOfGCThreads")
     conf.set(SERIALIZER, "org.apache.spark.serializer.KryoSerializer")
-    conf.set(KRYO_USER_REGISTRATORS, classOf[MyRegistrator].getName)
+    conf.set(KRYO_USER_REGISTRATORS, Seq(classOf[MyRegistrator].getName))
     conf.set(KRYO_USE_POOL, usePool)
 
     if (sc != null) {

core/src/test/scala/org/apache/spark/serializer/KryoSerializerDistributedSuite.scala

@@ -29,7 +29,7 @@ class KryoSerializerDistributedSuite extends SparkFunSuite with LocalSparkContex
   test("kryo objects are serialised consistently in different processes") {
     val conf = new SparkConf(false)
       .set(config.SERIALIZER, "org.apache.spark.serializer.KryoSerializer")
-      .set(config.Kryo.KRYO_USER_REGISTRATORS, classOf[AppJarRegistrator].getName)
+      .set(config.Kryo.KRYO_USER_REGISTRATORS, Seq(classOf[AppJarRegistrator].getName))
       .set(config.TASK_MAX_FAILURES, 1)
       .set(config.BLACKLIST_ENABLED, false)
 

core/src/test/scala/org/apache/spark/serializer/KryoSerializerSuite.scala

@@ -42,7 +42,7 @@ import org.apache.spark.util.ThreadUtils
 
 class KryoSerializerSuite extends SparkFunSuite with SharedSparkContext {
   conf.set(SERIALIZER, "org.apache.spark.serializer.KryoSerializer")
-  conf.set(KRYO_USER_REGISTRATORS, classOf[MyRegistrator].getName)
+  conf.set(KRYO_USER_REGISTRATORS, Seq(classOf[MyRegistrator].getName))
   conf.set(KRYO_USE_UNSAFE, false)
 
   test("SPARK-7392 configuration limits") {
@@ -313,7 +313,7 @@ class KryoSerializerSuite extends SparkFunSuite with SharedSparkContext {
     import org.apache.spark.SparkException
 
     val conf = new SparkConf(false)
-    conf.set(KRYO_USER_REGISTRATORS, "this.class.does.not.exist")
+    conf.set(KRYO_USER_REGISTRATORS, Seq("this.class.does.not.exist"))
 
     val thrown = intercept[SparkException](new KryoSerializer(conf).newInstance().serialize(1))
     assert(thrown.getMessage.contains("Failed to register classes with Kryo"))
@@ -412,7 +412,7 @@ class KryoSerializerSuite extends SparkFunSuite with SharedSparkContext {
     val ser = new KryoSerializer(new SparkConf).newInstance().asInstanceOf[KryoSerializerInstance]
     assert(ser.getAutoReset)
     val conf = new SparkConf().set(KRYO_USER_REGISTRATORS,
-      classOf[RegistratorWithoutAutoReset].getName)
+      Seq(classOf[RegistratorWithoutAutoReset].getName))
     val ser2 = new KryoSerializer(conf).newInstance().asInstanceOf[KryoSerializerInstance]
     assert(!ser2.getAutoReset)
   }
@@ -443,7 +443,7 @@ class KryoSerializerSuite extends SparkFunSuite with SharedSparkContext {
       .set(KRYO_REFERENCE_TRACKING, referenceTracking)
       .set(KRYO_USE_POOL, usePool)
     if (!autoReset) {
-      conf.set(KRYO_USER_REGISTRATORS, classOf[RegistratorWithoutAutoReset].getName)
+      conf.set(KRYO_USER_REGISTRATORS, Seq(classOf[RegistratorWithoutAutoReset].getName))
     }
     val ser = new KryoSerializer(conf)
     val serInstance = ser.newInstance().asInstanceOf[KryoSerializerInstance]
@@ -530,7 +530,7 @@ class KryoSerializerSuite extends SparkFunSuite with SharedSparkContext {
 
 class KryoSerializerAutoResetDisabledSuite extends SparkFunSuite with SharedSparkContext {
   conf.set(SERIALIZER, classOf[KryoSerializer].getName)
-  conf.set(KRYO_USER_REGISTRATORS, classOf[RegistratorWithoutAutoReset].getName)
+  conf.set(KRYO_USER_REGISTRATORS, Seq(classOf[RegistratorWithoutAutoReset].getName))
   conf.set(KRYO_REFERENCE_TRACKING, true)
   conf.set(SHUFFLE_MANAGER, "sort")
   conf.set(SHUFFLE_SORT_BYPASS_MERGE_THRESHOLD, 200)

core/src/test/scala/org/apache/spark/serializer/SerializerPropertiesSuite.scala

@@ -52,7 +52,7 @@ class SerializerPropertiesSuite extends SparkFunSuite {
 
   test("KryoSerializer does not support relocation when auto-reset is disabled") {
     val conf = new SparkConf().set(KRYO_USER_REGISTRATORS,
-      classOf[RegistratorWithoutAutoReset].getName)
+      Seq(classOf[RegistratorWithoutAutoReset].getName))
     val ser = new KryoSerializer(conf)
     assert(!ser.newInstance().asInstanceOf[KryoSerializerInstance].getAutoReset())
     testSupportsRelocationOfSerializedObjects(ser, generateRandomItem)

sql/core/src/test/scala/org/apache/spark/sql/DatasetSerializerRegistratorSuite.scala

@@ -34,7 +34,7 @@ class DatasetSerializerRegistratorSuite extends QueryTest with SharedSparkSessio
 
   override protected def sparkConf: SparkConf = {
     // Make sure we use the KryoRegistrator
-    super.sparkConf.set(KRYO_USER_REGISTRATORS, TestRegistrator().getClass.getCanonicalName)
+    super.sparkConf.set(KRYO_USER_REGISTRATORS, Seq(TestRegistrator().getClass.getCanonicalName))
   }
 
   test("Kryo registrator") {