conformance: add GRPCRoute request mirror test (#3514)

[Srujan-rai]

GRPCRoute already supports the RequestMirror filter (Extended conformance, apis/v1/grpcroute_types.go), but there were no conformance tests to verify implementations support it correctly.

This PR adds end-to-end conformance coverage by:

• Adding SupportGRPCRouteRequestMirror (GRPCRouteRequestMirror) as a new
  Extended feature in pkg/features/grpcroute.go, following the GEP-2162
  naming convention.
• Adding conformance/utils/grpc/mirror.go with ExpectMirroredRequest, which
  reads mirror backend pod logs and matches the fully qualified gRPC method
  name — the same approach used by http.ExpectMirroredRequest.
• Adding conformance/tests/grpcroute-request-mirror.{go,yaml}: a GRPCRoute
  that routes Echo requests to grpc-infra-backend-v1 and mirrors them to
  grpc-infra-backend-v2.
• Patching the gRPC echo server (conformance/echo-basic/grpc/grpc.go) to log
  the fully qualified method name on each received request
  ("Echoing back gRPC request made to <method> to client"), enabling
  log-based mirror verification.

Which issue(s) this PR fixes:

Fixes #3514

Does this PR introduce a user-facing change?:

Added GRPCRouteRequestMirror conformance test. Implementations that support
the RequestMirror filter on GRPCRoute can now declare                                                                                                       
SupportGRPCRouteRequestMirror as a supported feature.

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• ✅ login: Srujan-rai / name: srujan-rai (56e8a72)

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[snorwin]

@Srujan-rai thanks for the PR!

Could you help clarify how this differs from what's proposed in #4060?

Also, were you able to validate the new tests against a Gateway API implementation? If so, could you let us know which one?