[services][identity] grpc scaffolding for the identity service

Summary:
added a bare-bones implementation of the identity service defined in identity.proto

Depends on D3573

Test Plan: ran cargo build && cargo run, then used BloomRPC to make calls to the server. got a successful response.

Reviewers: jimpo, karol-bisztyga

Reviewed By: jimpo

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3578