Script: ulong support via Fields API

[stu-elastic]

Exposes unsigned long via the fields API.

Users must choose how to consume unsigned longs, either as regular java longs, where
the upper half of the range is treated as negative, or as BigIntegers.

Fields API now requires explicit typing until we do some work in painless work to allow duck
typing.

script_fields is not yet implemented with the fields API.

field('ul').asBigIntegerField().getValue(BigInteger.valueOf(1000))
field('ul').asBigInteger(BigInteger.valueOf(1000))
field('ul').asLongField().getValue(1000L)
field('ul').asLong(1000L)

Fixes: #64361

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[jdconrad]

This looks really good to me. One minor request - otherwise, I will leave the overall design review to @rjernst.

[stu-elastic]

closed in favor of #76519