Update wasm-shim to dispatch on ActionType

## Description

Update the wasm-shim to dispatch actions based on the `ActionType` discriminator field. When `ActionType` is set, the shim uses it to determine how to process the action. When `ActionType` is empty, the shim falls back to the existing `ServiceName`-based dispatch for backwards compatibility.

Action type dispatch:
- `grpc_method` — invoke the registered upstream gRPC method, evaluate Intention
- `allow` — evaluate Intention against request attributes, deny if false
- `add_headers` — evaluate HeadersToAdd CEL expression, add resulting headers to response
- `with_response_code` — override the HTTP response status code

## Acceptance Criteria

- [ ] Wasm-shim dispatches on `ActionType` when present
- [ ] Falls back to `ServiceName`-based dispatch when `ActionType` is empty
- [ ] All four action types implemented
- [ ] Unit tests

## References

- Design: [extensions-SDK-action-pipeline-design.md](https://github.com/Kuadrant/kuadrant-operator/blob/action-pipeline-proposal/doc/proposals/extensions-SDK-action-pipeline-design.md)
- Parent: #1898