Support registering bundle classes outside of autodiscovery

Tests can be arranged if we can agree on implementation/direction.

Wondering if this can be incorporated in to the core MR which now does its own discovery outside of plugins - https://git.drupalcode.org/project/drupal/-/merge_requests/11545

In its current state it searches for an Entity subdir in all container.namespaces. Would be worth seeing if it can be made to work with this use case.

container.namespaces includes only Drupal extensions.

The desire for this issue is to include classes defined in vendor.

Is there a way we can refactor the core MR to do this? E.g. maybe CoreServiceProvider passes a container param with some hints of where to scan, the custom module can modify the container param and then the BundleClassCollectorPass can find them.

Theres been discussions of if or how to scan vendor for plugins before, I dont have references on hand.

I think core would want to look at composer autoload namespace and directories and scan them; whereas the change here is a simple list of which classes to include. No directory scans.

For selfish reasons I dont really have time on hand to look through core, as this is very much a side thing of what I'm working on for PNX. I'd prefer a quick merge but otherwise will advise to use a patch in the meantime.

We'll do it outside of BCA for now, but may revisit due to need for a flexible weighting system.