Add ActionType discriminator to wasm Action struct

[philbrookes]

Description

Add an explicit ActionType discriminator field to the wasm Action struct in internal/wasm/types.go. The wasm-shim dispatches based on this field value rather than inspecting which fields are present.

New fields on Action:

• ActionType — grpc_method, allow, add_headers, with_response_code
• Intention — CEL expression
• ActionMethod — registered method name
• HeadersToAdd — CEL expression evaluating to a map of headers
• NewResponseCode — HTTP status code

Existing actions (auth, ratelimit, tracing) do not set ActionType — the wasm-shim falls back to ServiceName-based dispatch when empty, preserving backwards compatibility.

Acceptance Criteria

• ActionType field and constants added to Action struct
• New pipeline action fields added (Intention, ActionMethod, HeadersToAdd, NewResponseCode)
• Existing actions unaffected (backwards compatible)
• Unit tests

References

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