[keyserver] add gRPC client module to keyserver

Summary:
this module loads the identity.proto file and creates a client for the Identity service.

Depends on D5573

Test Plan: imported the identityClient in keyserver.js and successfully called the getUserPublicKey RPC

Reviewers: tomek, jon, bartek, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5585