Use CallableResolver for form callbacks

Created a quick proof of concept.

All form callbacks go through FormState::getCallback(), which is nice in that it gives us a single place to adjust, but FormState isn't something we can inject, so we have to use Drupal::service() which I absolutely expect will break some unit tests.

Might want to deprecate this and add something else.

Yep a lot of container not initialized yet failures.

kim.pepper → made their first commit to this issue’s fork.

Added 📌 Move file form hooks to FileFormHooks class Active for the hooks cleanup mentioned above.

Fixed the failing test by handling the exception. Not sure if this is the right place to handle it.