[Identity] Add authenticated gRPC service

Summary:
Add another grpc service for authenticated requests.
This differs from the existing service by having an interceptor
which will fail if authentication did not succeed.

Test Plan:
N/A. Implementation in later diff.
This is meant to be more of a design review.

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8434