Add support for a first-party client app to call into Khoj (Part 1)

[sabaimran]

Incoming

• Add a new ClientApplication model, consisting of a client ID and client secret which allows an external first party application to call into the Khoj server and access a particular user's data (and get access to all khoj chat features!)
  • Initial implementation uses phone numbers as the identifier for users
• Part 2 will allow users to specify their phone numbers to associate their Khoj account with a number. Otherwise, orphaned phone numbers can exist which are connected to first party applications.

[sabaimran]

Updated based on feedback in https://github.com/khoj-ai/flint/pull/14 to make the client secret come through the authorization header.