[cfid-364] Add remote authentication endpoint

Provides a more robust endpoint for the login server to call,
getting back a clear response code indicating that authentication
has succeeded (200). The client should also read the user information
from the JSON response (currently just the username).
[#40539283].

Change-Id: I9f10a8ea151098416abacdb3d2835a5125a3ffb7
Merge "[cfid-364] Add remote authentication endpoint" into develop