[services][blob] Add http_port config option

Summary:
A preparation to introduce HTTP server (ENG-3520).

Renamed --port CLI option to --grpc-port and added another --http-port option. Both gRPC and HTTP servers will be able to run concurrently.

Also changed the type to u16 to make it compatible with actix-web API I'm going to use.

Depends on D7440

Test Plan: Build and run the blob service, play with --grpc-port CLI option to ensure it changes ports. The new --http-port will be tested when HTTP server is introduced (two diffs later)

Reviewers: marcin, michal, jon, varun

Reviewed By: jon, varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7441