[services][blob] Scaffold gRPC server

Summary:
Boilerplate for proto interface. The server listens on 50051 port,
this is the default in current C++ implementation for all services. In subsequent diffs
I'll add possibility to configure this.

Depends on D5676

Test Plan:
Run the service. Client should be able to send requests, server should send
"unimplemented" error messages to client.

Reviewers: varun, jon, tomek, marcin

Reviewed By: varun, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5677