[services][identity] implement generate_nonce RPC in new client_service.

Summary: I just copied the existing implementation from the old service.

Test Plan:
this is really just copy and paste of code that has been tested,
but it gets tested again two diffs later in the stack

Reviewers: jon

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7213