Adding support for generic front logout channel

[StarAurryon]

All Submissions:

• [x] Have you followed the plugin Contributing guideline?
• [x] Does your code follow the WordPress' coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Implementing standard OpenID front logout channel as per https://openid.net/specs/openid-connect-frontchannel-1_0.html.

Closes #336 .

How to test the changes in this Pull Request:

1. Fill the revoke endpoint in the settings of the plugin;
2. Do a GET on /wp-admin/admin-ajax.php?action=openid-connect-logout&sid=TOKEN_SID&iss=TOKEN_ISS with the browser of a connected user;
3. The user should be disconnected.

Changelog entry

• Adding revoke OpenID endpoint in the settings;
• Adding openid-connect-logout endpoint to perform the front logout (iframe rendered by the client).

[timnolte]

@StarAurryon also, this PR doesn't pass code quality checks so that also needs to be addressed. Your PR doesn't reference any open issues/feature requests if there are any open issues that pertain to this change they should be referenced in your PR. Thanks!

[StarAurryon]

My apologies. I was not able to set up the proper testing framework at first (requiring composer 1) and I had to dig a bit further to find what was missing.

Hope this is better now.