[identity] log out user RPC

Summary:
calling this RPC will clear a device's access token and keys from DDB

Depends on D8500

Test Plan: called the RPC from kreya and verified that my device's keys and token were removed from DDB

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8501