Expose redirect_url attribute for OAuth

[tcompa]

Ref:

• Added GitHub login (OAuth2) fractal-web#333

Todo:

• Introduce OAuthClientConfig.REDIRECT_URL optional attribute
• If this is not None, use it as a redirect_url attribute for fastapi_users.get_oauth_router (ref https://github.com/fastapi-users/fastapi-users/blob/master/fastapi_users/router/oauth.py and https://frankie567.github.io/httpx-oauth/fastapi/#oauth2authorizecallback). Quote: redirect_url: Optional[str]: Full URL to the callback route..
• Update docs.