Set OAuth scopes at init time

[XaviTorello]

Will be great if scopes can be defined at init time and reused when needed.

Some methods like get_authorization_url needs to receive the scope as a param; our idea is to directly use the configured scopes for the current backend to avoid security issues (scopes should be received as an endpoint param, so everyone can change the requested scopes "in our name").

We've a working PoC, in short we'll submit a PR providing compatibility with current approach.

Related with fastapi-users/fastapi-users#664