Fixed #823 (#824)

* Added a failing test for the multi-oauth-router issue

* Fixed the #823 regression.

Using a regex for the backend name validation instead of an enum.

* Fixed formatting errors

* Moved the `AuthenticationBackendName` enum to `Authenticator`

This prevents an issue with OpenAPI schema generation caused by two
endpoints accepting a parameter with a duplicate name.

fastapi_users/authentication/__init__.py

@@ -1,3 +1,4 @@
+import enum
 import re
 from inspect import Parameter, Signature
 from typing import Callable, Optional, Sequence
@@ -42,6 +43,7 @@ class Authenticator:
     """
 
     backends: Sequence[BaseAuthentication]
+    backends_enum: enum.Enum
 
     def __init__(
         self,
@@ -49,6 +51,10 @@ class Authenticator:
         get_user_manager: UserManagerDependency[models.UC, models.UD],
     ):
         self.backends = backends
+        self.backends_enum = enum.Enum(  # type: ignore
+            "AuthenticationBackendName",
+            {backend.name: backend.name for backend in backends},  # type: ignore
+        )
         self.get_user_manager = get_user_manager
 
     def current_user(