Implement io.jwt.verify_eddsa builtin

[sspaink]

Missing builtin

• io.jwt.verify_eddsa

Context

This OPA builtin is not implemented in the SDK. It is exercised by the Rego compliance suite and currently raises FunctionNotFoundError.

Sibling of the existing io.jwt.verify_* builtins; belongs in the opa-builtins-token module.

Implementation notes

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

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