[LW] OAuth SSO GitLab

Wordpress OAuth A bare-minimum Wordpress plugin to allow single-sign-on via OAuth. Tested with Amazon Cognito but should work with any OAuth compliant server. Originally forked from the wordpress plugin (oauth-client-for-user-authentication)[https://wordpress.org/plugins/oauth-client-for-user-authentication/], as when testing that plugin I found many security vulnerabilities so I decided to use it as a base and strip it back as much as possible to make the code easy to audit and understand. The plugin scope is designed to be deliberately very simple. It supports: - Adding a button to the login form to begin SSO - Authenticating with a single Identity Provider - Automatically redirecting /wp-login.php to begin the OAuth process with the IDP - Finding a user by any attribute returned by your IDP, against any wordpressusers column - Redirecting a user to your IDP's logout endpoint after logging out of Wordpress It does not, and will not, support: - Editing plugin settings from the wp-admin interface - Registering/making new users for IDP users - Syncing from / authenticating to LDAP - Multiple Identity Providers - OpenID Connect It does not currently support, but suppo