[GRPCRoute] Investigate and select gRPC backend for testing

[averevki]

Summary

Investigate and select a gRPC-capable backend image for use in E2E tests, replacing httpbin for gRPC test scenarios.

Motivation

GRPCRoute E2E tests (#886) require a gRPC backend deployed in Kubernetes. The backend must support gRPC reflection (for dynamic clients without compiled protobuf stubs) and return enough information to validate request metadata in tests.

Candidates

The GRPCRoute support RFC lists the following candidates:

Image	Pros	Cons
grpcbin	Purpose-built echo server, supports reflection, well-known in gRPC community	May not be actively maintained
Fortio	Multi-protocol (HTTP + gRPC), load testing capable, actively maintained	Heavier than needed
Istio echo	Multi-protocol, used by Istio's own tests	Tied to Istio ecosystem

Tasks

• Evaluate each candidate image against testsuite requirements
• Verify selected image supports gRPC reflection
• Verify selected image can echo back request metadata (headers/gRPC metadata) for test assertions
• Verify selected image works with both Istio and Envoy Gateway providers
• Implement GRPCBackend class in testsuite/backend/grpc.py extending Backend
• Testsuite configuration (config/settings.yaml) updated with gRPC backend image setting
• Document the selection rationale

References

• Parent issue: E2E tests for GRPCRoute support in Kuadrant policies #886
• GRPCRoute Support RFC (Task 2 - gRPC backend image)
• Existing Backend interface
• Existing Httpbin backend