Script: ulong via fields API (elastic#76519)

* Script: ulong via fields API

Exposes unsigned long via the fields API.

Unsigned longs default to java signed longs.  That means the upper range
appears negative.  Consumers should use `Long.compareUnsigned(long, long)`
`Long.divideUnsigned(long, long)` and `Long.remainderUnsigned(long, long)`
to correctly work with values known to be unsigned long.

Alternatively, users may treat the unsigned long type as `BigInteger` using
the field API, `field('ul').as(Field.BigInteger).getValue(BigInteger.ZERO)`.
```
field('ul').as(Field.BigInteger).getValue(BigInteger.valueOf(1000))
field('ul').getValue(1000L)
```

This change also implements the beginning of the converters for the fields
API.  The following conversions have been added:
```
ulong <-> BigInteger
long <-> BigInteger
double -> BigInteger
String (parsed as long or double) -> BigInteger
double -> long
String (parsed as long or double) -> long
Date (epoch milliseconds) -> long
Nano Date (epoch nanoseconds) -> long
boolean (1L for true, 0L for false) -> long
```

modules/lang-painless/src/main/resources/org/elasticsearch/painless/org.elasticsearch.script.fields.txt

@@ -10,10 +10,18 @@
 
 # API
 class org.elasticsearch.script.Field {
+  org.elasticsearch.script.Converter BigInteger
+  org.elasticsearch.script.Converter Long
   String getName()
   boolean isEmpty()
   List getValues()
   def getValue(def)
+  double getDouble(double)
+  long getLong(long)
+  org.elasticsearch.script.Field as(org.elasticsearch.script.Converter)
+}
+
+class org.elasticsearch.script.Converter {
 }
 
 class org.elasticsearch.script.DocBasedScript {