perf(grpcrouter): Investigate alternatives to proto.Merge

[testinginprod]

In #17921

Since the Handler signature is: func(ctx context.Context, req, resp protoiface.Messagev1) error
And the gRPC method handler is func(ctx context.Context, decoderFunc func(msg any) error, handler func(ctx context.Context, req any) (resp any, err error)

We need to decode req from Handler into msg from decoderFunc, to do so we use merging, this also brings in support and safety with respect to synthetic messages like dynamicpb.

Then since resp  is passed in by Handler and the grpc method handler returns any we need to copy the response from the grpc method handler into resp from Handler, to do so we use proto.Merge again, which as for the former case allows for support of synthetic messages.

Investigate if there is a more performant way of doing so.