GCP: Bigtable data plane (ReadRows/MutateRow) is gRPC-only, unreachable over the HTTP wire server

[NitinKumar004]

Bigtable admin (REST) works; the data plane (ReadRows/MutateRow/etc.) is gRPC-only and cannot be reached over cloudemu's HTTP wire server. Documented deferral; needs a gRPC transport seam.