Add GRPCRoute coverage in request-mirror tests

[LiorLieberman]

What would you like to be added:
GRPCRoute coverage in http-request-mirror tests

Why this is needed:
cause.. why not? :)

This is to improve coverage for GRPCRoute conformance tests.

We should probably associate this with a different supportedFeature to allow gradual/selective support. Thoughts?

/cc @mikemorris @kflynn @robscott

[mikemorris]

I think it would be a new group, i.e. grpc-request-mirror, but we could factor out any common logic and yea we should do this, probably just a matter of prioritization/contribution and review bandwidth.