Convert Filter plugin discovery to attributes

Time for feedback.

Added some suggestions mostly the same as on the Mail issue but didn't get through the whole MR.

Couple nits but also I don't think the last few params should be nullable.

FWIW I opened 📌 Create enum for FilterInterface constants Needs work which is relevant, but not a blocker, for this issue.

I really wanted to RTBC this, but there are a few incorrect default values here that would cause regressions after contrib adopts this.

This is 99% ready, nice job! 🥳

Looks good! :)

Committed and pushed 1d07b82287 to 11.x and 1e89b0ca64 to 10.3.x. Thanks!

Thanks, that really paves the path for 📌 Create enum for FilterInterface constants Needs work 👍

Automatically closed - issue fixed for 2 weeks with no activity.

quietone → closed merge request !6611