[protos] Add RPC to log in existing device

Summary:
Adds Existing device login RPC from ENG-6828 to the proto.

Depends on D11435

Test Plan: Only proto changes here + stub code. Everything should compile.

Reviewers: ashoat, varun

Reviewed By: ashoat, varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11465