Implement URL query builtins (urlquery.*)

Missing builtins

• urlquery.encode
• urlquery.decode
• urlquery.encode_object
• urlquery.decode_object

Context

These OPA builtins are not implemented in the SDK. Most are exercised by the Rego compliance suite (opa-evaluator/src/test/resources/compliance/.../TestData) and currently raise FunctionNotFoundError; implementing them makes those cases pass.

Implementation notes

Register each builtin with an @OpaBuiltin-annotated static method (see io.github.open_policy_agent.opa.ast.builtin.impls). Core builtins are listed in BuiltinRegistry.BUILTIN_CLASSES; domain-specific ones live in an opa-builtins-* module exposed via the BuiltinProvider SPI (e.g. RegexBuiltins, CidrBuiltins, SemverBuiltins).

Reference: https://www.openpolicyagent.org/docs/policy-reference/#built-in-functions

Start with BuiltinRegistry.BUILTIN_CLASSES and the io.github.open_policy_agent.opa.ast.builtin.impls package, then compare RegexBuiltins, CidrBuiltins, and SemverBuiltins for registration patterns. Review the compliance fixtures under opa-evaluator/src/test/resources/compliance/.../TestData. Done means all four urlquery builtins are exposed through the appropriate builtin provider and the affected compliance cases no longer raise FunctionNotFoundError.