APIv4 - Use event instead of hardcoded list to resolve entityName to className

APIv4 - Use event instead of hardcoded list to resolve entityName to className

Typically an APIv4 entity name and class name are the same,
e.g. the "Contact" entity has the class `Civi\Api4\Contact`.
However, there are a few oddballs (CiviCase, CustomValue) and other extensions
may introduce their own oddball entities; this event allows them to do so.