Add OAuth2 authentication provider

Add OAuth2 authentication provider This change depends on the changes in PR #940. It uses the new method introduced in that PR to identify the resource being requested and get the plugin definition to read the oauth2 server and scope to use for authorizing the request. This would probably need to be changed once #940 is merged or a better method to access the request being processed is identifed (as mentioned in #939). I am creating this PR in hope of clarifying the changes in PR #940.