PB-1109: Add PUT endpoint to update user

[asteiner-swisstopo]

Adds PUT endpoint users/{username} to the the REST API of service-control that allows you to update a user.

Summary of changes:

1. Add new PUT endpoint users/{username} that returns HTTP status code

   • 200 (OK) if the user has been updated successfully
   • 404 (Not Found) if there is no user with the given username
   • 500 (Internal Server Error) if there is an inconsistency with Cognito or if there is no provider with the given ID
     ⚠️ This is is the only difference in behaviour to the DELETE in Add DELETE endpoint for users #34
   • 503 (Service Unavailable) if Cognito cannot be reached

   ℹ️ This uses PUT and not PATCH because we deem it good enough to start with and it's supposedly easier to implement. It's a bit annoying maybe that the user has to specify all fields, even those she/he does not want to change. But then again they can those values via the GET.

2. Rename cognito.utils.user.update_user to update_cognito_user to put more clearly indicate that this is about Cognito and not about the User model in Django. See the discussion in Add DELETE endpoint for users #34.

Example payload to PUT users/{username}:

{
    "username": "dude",
    "first_name": "Jeff",
    "last_name": "Bridges",
    "email": "[email protected]",
    "provider_id": 123,
}

Example response if provider does not exist:

{
    "code": 500,
    "description": "Provider does not exist"
}

[schtibe]

Just small comments, looking good otherwise 👍