Implement providers.aws.sign_req builtin

[sspaink]

Missing builtin

• providers.aws.sign_req

Context

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

Signs an HTTP request with AWS SigV4.

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