[SPARK-23901][SQL] Add masking functions

## What changes were proposed in this pull request?

The PR adds the masking function as they are described in Hive's documentation: https://cwiki.apache.org/confluence/display/Hive/LanguageManual+UDF#LanguageManualUDF-DataMaskingFunctions.
This means that only `string`s are accepted as parameter for the masking functions.

## How was this patch tested?

added UTs

Author: Marco Gaido <marcogaido91@gmail.com>

Closes #21246 from mgaido91/SPARK-23901.

sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/MaskExpressionsUtils.java

@@ -0,0 +1,80 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.expressions;
+
+/**
+ * Contains all the Utils methods used in the masking expressions.
+ */
+public class MaskExpressionsUtils {
+  static final int UNMASKED_VAL = -1;
+
+  /**
+   * Returns the masking character for {@param c} or {@param c} is it should not be masked.
+   * @param c the character to transform
+   * @param maskedUpperChar the character to use instead of a uppercase letter
+   * @param maskedLowerChar the character to use instead of a lowercase letter
+   * @param maskedDigitChar the character to use instead of a digit
+   * @param maskedOtherChar the character to use instead of a any other character
+   * @return masking character for {@param c}
+   */
+  public static int transformChar(
+      final int c,
+      int maskedUpperChar,
+      int maskedLowerChar,
+      int maskedDigitChar,
+      int maskedOtherChar) {
+    switch(Character.getType(c)) {
+      case Character.UPPERCASE_LETTER:
+        if(maskedUpperChar != UNMASKED_VAL) {
+          return maskedUpperChar;
+        }
+        break;
+
+      case Character.LOWERCASE_LETTER:
+        if(maskedLowerChar != UNMASKED_VAL) {
+          return maskedLowerChar;
+        }
+        break;
+
+      case Character.DECIMAL_DIGIT_NUMBER:
+        if(maskedDigitChar != UNMASKED_VAL) {
+          return maskedDigitChar;
+        }
+        break;
+
+      default:
+        if(maskedOtherChar != UNMASKED_VAL) {
+          return maskedOtherChar;
+        }
+        break;
+    }
+
+    return c;
+  }
+
+  /**
+   * Returns the replacement char to use according to the {@param rep} specified by the user and
+   * the {@param def} default.
+   */
+  public static int getReplacementChar(String rep, int def) {
+    if (rep != null && rep.length() > 0) {
+      return rep.codePointAt(0);
+    }
+    return def;
+  }
+}

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -432,6 +432,14 @@ object FunctionRegistry {
     expression[ArrayRepeat]("array_repeat"),
     CreateStruct.registryEntry,
 
+    // mask functions
+    expression[Mask]("mask"),
+    expression[MaskFirstN]("mask_first_n"),
+    expression[MaskLastN]("mask_last_n"),
+    expression[MaskShowFirstN]("mask_show_first_n"),
+    expression[MaskShowLastN]("mask_show_last_n"),
+    expression[MaskHash]("mask_hash"),
+
     // misc functions
     expression[AssertTrue]("assert_true"),
     expression[Crc32]("crc32"),