Support PHP attributes for KeyProvider plugins

ptmkenny → closed merge request !31

The current MR ONLY adds attribute support without actually adding the attribute to the plugins. The reason is because attributes will break on PHP < 8.1 and don't work with Drupal < 10.2. Since this module supports older versions of Drupal, addition of attributes to plugins should be postponed, but the Attribute itself can be committed now so that downstream modules (with stricter version requirements) can start using it.