Converts BackedEnum to string in EventFilterService

Problem/Motivation

The flowdrop_trigger module's event type plugin definitions can return a BackedEnum for the "category" key (this is common in Drupal 11 plugins using PHP 8.1+ enums). When sprintf receives a BackedEnum object instead of a string, PHP throws a fatal error because enums can't be implicitly cast to string.

So, any trigger event whose plugin definition uses an enum for category would crash when getCategoryConfigName() is called.