[keyserver] add source field to LogInRequest

Summary:
Add source param on the server LogInRequest type. (task)

Test Plan:

1. flow check
2. Test login with different sources (from the native, web, on session recovery when cookie expired) and make sure provided source is correct.
3. Make sure it works for older native clients (e.g. pass source as undefined)

Reviewers: tomek, atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5260