Fully connect the /users/token endpoint component pieces

[chouinar]

Summary

We want to connect all of the pieces of the implementation together in this ticket. That is:

• Using the endpoint written in Create POST /users/token stub endpoint #2673
• It should take the token it received, and pass it to the logic created in Add logic to process and validate login.gov JWT #2721
• The ID from this should be used to query the users table (by external_user_id) and create a user if doesn't exist
• We should then generate a JWT from this user using the logic in Setup logic to generate a JWT in our API #2808

Acceptance criteria

• Thorough API-wide unit tests written that hit many scenarios (happy path, and error)