[keyserver] allow source field in log_in endpoint validation

Summary:
We will need to pass the source field to the keyserver to distinguish between different log-in scenarios (task).
This diff adds this field to the strict interface but allows an undefined source (which will happens on logging from the login form).
A proper type for login from form will be added later on and this field will become mandatory.

Test Plan:

1. Modify the code to pass the source field.
2. Check if validation pass for both undefined (e.g. login from form) and defined source (e.g. cookie expired).

Reviewers: tomek, atul, abosh

Reviewed By: tomek

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5227