Allow sharing of the job and artifact service address.

GRPC can serve multiple services off of the same address. It would be ideal to allow serving the artifact service off the same port as the job service, especially when firewalls and port forwarding are involved. Of particular note, the artifact staging service address provided in the job preparation response could have a special value indicating "use the job server address" as the job server itself may be incapable as providing an address accessible to the client. 

[~mxm] [~angoenka]

Imported from Jira [BEAM-6728](https://issues.apache.org/jira/browse/BEAM-6728). Original Jira may contain additional context.
Reported by: robertwb.