[identity] Implement LogInExistingDevice RPC

Summary:
Implementation of the LogInExistingDevice RPC. Its purpose is to:

• verify nonce challenge
• check if device being logged in is on the device list
• generate access token for the device

Depends on D11465

Test Plan:

1. Registered a device - received a CSAT (1)
2. Called the LogInExistingDevice RPC - Received a CSAT (2)
3. Called VerifyUserAccessToken RPC for both CSATs - (2) is valid, (1) is no longer valid

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11466