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/models.py

@@ -1,5 +1,5 @@
 import uuid
-from typing import Optional, TypeVar
+from typing import List, Optional, TypeVar
 
 import pydantic
 from pydantic import BaseModel, EmailStr
@@ -19,7 +19,8 @@ class BaseUser(BaseModel):
 
     def create_update_dict(self):
         return self.dict(
-            exclude_unset=True, exclude={"id", "is_superuser", "is_active"}
+            exclude_unset=True,
+            exclude={"id", "is_superuser", "is_active", "oauth_accounts"},
         )
 
     def create_update_dict_superuser(self):
@@ -44,3 +45,28 @@ class BaseUserDB(BaseUser):
 
 
 UD = TypeVar("UD", bound=BaseUserDB)
+
+
+class BaseOAuthAccount(BaseModel):
+    """Base OAuth account model."""
+
+    id: Optional[str] = None
+    oauth_name: str
+    access_token: str
+    expires_at: int
+    refresh_token: Optional[str] = None
+    account_id: str
+    account_email: str
+
+    @pydantic.validator("id", pre=True, always=True)
+    def default_id(cls, v):
+        return v or str(uuid.uuid4())
+
+    class Config:
+        orm_mode = True
+
+
+class BaseOAuthAccountMixin(BaseModel):
+    """Adds OAuth accounts list to a User model."""
+
+    oauth_accounts: List[BaseOAuthAccount] = []