Accessing Bing Webmaster APIs using OAuth 2.0

Problem/Motivation

There is another way to get access to the Bing Webmaster APIs and it is OAuth 2.0. It is recommended by Bing.

Remaining tasks

1. Add new settings to the /admin/config/services/bing-index-api with select of methods (API key or OAuth 2.0)
2. Implement new access for OAuth 2.0 method.