Define action types and Pipeline interface

[philbrookes]

Description

Define the action type structs and Pipeline interface in pkg/extension/types/.

Action types:

• GRPCMethodAction (request phase) — invokes a registered gRPC method, evaluates response with Intention
• AllowAction (request phase) — allows/denies based on request attributes only
• AddHeadersAction (response phase) — adds headers via a CEL expression
• WithResponseCodeAction (response phase) — overrides the HTTP status code

RequestAction and ResponseAction are separate interfaces because request-phase actions can deny the request (failed Intention stops it), while response-phase actions cannot deny.

Acceptance Criteria

• RequestAction and ResponseAction interfaces defined
• GRPCMethodAction, AllowAction, AddHeadersAction, WithResponseCodeAction structs defined
• ActionType enum with grpc_method, allow, add_headers, with_response_code values
• Pipeline interface with OnRequest(ctx, ...RequestAction) error and OnResponse(ctx, ...ResponseAction) error
• Unit tests

References

• Design: extensions-SDK-action-pipeline-design.md
• Parent: Extension SDK Action Pipeline #1898