Customize register-user endpoint, to make creation of user and settings simultaneous

[tcompa]

We are starting by simply using the on_after_register hook, but this requires that first we create a valid user and then we associate it to a settings entry -> therefore we cannot make the FK required.

If we customize SQLModelUserDatabaseAsync.create, we could create the two DB objects at the same time.

Ref:

• TBD - Reduce dependency on fastapi-users features, when we need to customize them #1739

---

We have at least two options:

1. Add attribute SQLModelUserDatabaseAsync.user_settings_model, and customize SQLModelUserDatabaseAsync.create so that it creates both the user and their settings.
2. Rewrite the POST /auth/register/ endpoint without using fastapi-users or SQLModelUserDatabaseAsync logics - or maybe by only using some of them.

[tcompa]

A good attempt at this was #1789.

For the moment, we will keep the UserOAuth.user_settings_id foreign key optional, even though it will always be present in practice. In a later update, we'll try to fully integrate the user-register and oauth-related endpoints from fastapi-users into fractal-server.

We decided to postpone this task mostly due to several other ongoing updates - but this is ready for further work.

cc @ychiucco