Expand OAuth2Manager

Problem/Motivation

Building up on #2939922: Expand OAuth2Manager, we need to expand OAuth2Manager for Social Auth.

Proposed resolution

(Description of the proposed solution, the rationale behind it, and workarounds for people who cannot use the patch.)

Remaining tasks

Define methods getExtraDetails, requestEndPoint, getScopes, and getEndPoints

API changes

Child classes must implement getExtraDetails and requestEndPoint and pass the configuration object of the implementer to the parent constructor. Alternatively, the endpoints field in the settings form can be changed to 'endpoints' (most implementers call it 'api_calls') to avoid rewriting getEndPoints method.