Implement OAuth2 flow (#88)

* Move users router in sub-module

* Factorize UserRouter into EventHandlersRouter

* Implement OAuth registration/login router

* Apply isort/black

* Remove temporary pytest marker

* Fix httpx-oauth version in lock file

* Ensure ON_AFTER_REGISTER event is triggered on OAuth registration

* Add API on FastAPIUsers to generate an OAuth router

* Improve test coverage of FastAPIUsers

* Small fixes

* Write the OAuth documentation

* Fix SQL unit-tests by avoiding collisions in SQLite db files

fastapi_users/router/common.py

@@ -0,0 +1,35 @@
+import asyncio
+from collections import defaultdict
+from enum import Enum, auto
+from typing import Callable, DefaultDict, List
+
+from fastapi import APIRouter
+
+
+class ErrorCode:
+    REGISTER_USER_ALREADY_EXISTS = "REGISTER_USER_ALREADY_EXISTS"
+    LOGIN_BAD_CREDENTIALS = "LOGIN_BAD_CREDENTIALS"
+    RESET_PASSWORD_BAD_TOKEN = "RESET_PASSWORD_BAD_TOKEN"
+
+
+class Event(Enum):
+    ON_AFTER_REGISTER = auto()
+    ON_AFTER_FORGOT_PASSWORD = auto()
+
+
+class EventHandlersRouter(APIRouter):
+    event_handlers: DefaultDict[Event, List[Callable]]
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.event_handlers = defaultdict(list)
+
+    def add_event_handler(self, event_type: Event, func: Callable) -> None:
+        self.event_handlers[event_type].append(func)
+
+    async def run_handlers(self, event_type: Event, *args, **kwargs) -> None:
+        for handler in self.event_handlers[event_type]:
+            if asyncio.iscoroutinefunction(handler):
+                await handler(*args, **kwargs)
+            else:
+                handler(*args, **kwargs)