Create enum for FilterInterface:TYPE_* constants

First pass, see what the test bot thinks. Still a few references to the legacy consts and some deprecation to do.

Have put in some @todos where I think we can trigger deprecations. We should also have a test module using the old constants in the plugin annotations.

Also adding 📌 Convert Filter plugin discovery to attributes Needs work as related.

Yes, please!

This looks fantastic!

I posted an idea for how to deal with the getFilterType() return type conundrum. Not 100% sure yet if it'll work, but it's a start :D

📌 Convert Filter plugin discovery to attributes Needs work landed, let's push this forward again?

Rebased on 11.x and updated the attribute to expect the enum instead of int. Hopefully we don't have to deprecate passing an int to in the attribute if we can get this out before a 10.3 release? Needs work to address the feedback from Wim.