This commit started with me trying to address cross-resource issue with
Microsoft Outlook and Graph endpoints and ended up being a plugin
rewrite.

Some of the changes include;

* Making getting Resource Owner Optional - this was especially necessary
  for Microsoft Email provider
* Added support for providers specifying custom Config class
* Extend League AccessToken so we can best manage Token object e.g
  getting resource owner attributes
* Added ability to decode jwt token to get resource owner info when
  getting resource owner is not an option (Thanks to M$)
* Updated default endpoints as needed

When adding a new instance of MicrosoftEmailOauth2 the core validates
data using default plugin config form instead of the custom one.

This commit addresses the issue temporarily by injecting resource owner
url for validation purpose only. In the upcoming version of core we will
accept either a form or vars - making it possible to support older
plugins until end of life is reached.