Scripting: Converters can adapt return values

[stu-elastic]

Add ability to explicitly coerce the return value from a script.

Runtime fields wants to avoid returning Object from the execute method
in each context. Instead, they will return an array of primitives, such as
long[].

However, it's convenient to allow a user to return a single primitive
type rather than allocating a length-one array.

To achieve this, an implementer can add explicit conversion functions to
a context with the signature:
public static <context-return-value> convertFrom<Suffix>(<any type>)

When a user returns a type other than the context return value, at
compile-time, painless will insert a call to their convertFrom method.

This commit is Phase 1 of this work. It handles explicit converters for
all painless types EXCEPT def type.

Refs: #59647

[elasticmachine]

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

[jdconrad]

Solid first step! Just as a note, when we address primitives I think it's better to do this as a standard cast from the type to the parameter of the conversion method and then a second step where the method is actually called.