Updated OAuth2 router generator for better OpenAPI docs (#793)

* Added OAuth2 authorize route response_model

* Changed OAuth2 "authorize" route backend param type.

On OAuth2 router creation, an enum is created for the `authentication_backend` parameter. This allows us to generate OpenAPI docs correctly.

* Reformatted with "make format"

* Updated OAuth2 authorize url docs

- /authorize now returns 422 instead of 400 for an invalid authentication backend

fastapi_users/models.py

@@ -79,3 +79,7 @@ class BaseOAuthAccountMixin(BaseModel):
     """Adds OAuth accounts list to a User model."""
 
     oauth_accounts: List[BaseOAuthAccount] = []
+
+
+class OAuth2AuthorizeResponse(BaseModel):
+    authorization_url: str