When testing CSM Observability, we discovered that the c++ xds interop client is not sending any payload with the `UnaryCall` RPCs so most of the metrics will have a value of 0.
Adding a payload to the xds interop client here.
We need this fix so that we can verify that the metrics are recording the right number of bytes being sent / received. So we need a non-trivial payload to be sent with the `UnaryCall` RPC between the xds interop client and server.
Closes#35545
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35545 from stanley-cheung:xds-client-payload 11be4e6f4f
PiperOrigin-RevId: 601596246
48961e8a2c