[shared/grpc_clients] Add verify_user_access_token()

Summary:
Add commonly needed function for services.

https://linear.app/comm/issue/ENG-4820

D9014

Test Plan:

cd shared/grpc_clients
cargo build

Reviewers: bartek, varun, kamil

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9015