Convert the field permissions types to plugins

Here's a start. I'd like to pull further permission-type-specific logic into the individual plugins, but the field report makes it difficult.

I've left the 'public' permission type hard coded since it basically means to ignore the field as far as this module is concerned, so firing up a plugin for that seemed like overkill.

That patch was missing an interface.

Not sure why these are failing.

These tests continue to be green locally.

The test fails have something to do with plugin discovery. I'm adding some kernel tests to try and figure out why these only fail on Drupal CI.

The issue seems to be that the CI can't find the plugins. I'd guess this has to do with overlooked case-sensitivity, but I can't see any. This patch changes how the custom plugin manager is declared (similar to the Filter plugin manager in core).

Adding a unit test to try and suss out the bizarre failures here. I am still unable to replicate these locally.

I think it was indeed a case-sensitivity issue. Can't believe I didn't catch this sooner.

Done and done.