Add configurable client-side keep alive sending

Add the configurable PING sending from the client to make the channel alive for the intermediate servers, like load balancers.

It's the followup task for the QTBUG-135049.

It could be that this mechanism do not solve the actual issue, since the nature of the disconnection is not completely investigated, but reference gRPC allows configuring the client-side keep alive sending, so should QtGrpc too.

Ref:

• https://github.com/grpc/grpc/blob/master/doc/keepalive.md
• https://grpc.io/docs/guides/keepalive/