Add migrate path for block plugin

Hey together,

I have added a draft MR to review the block entity migration to the new block plugin and settings.
Can anyone review this to see if anything works as expected?

Any feedback would be awesome.

I tested successfully a default migration from 1.x to 2.x.

Made some minor changes to the settings form so we are good to go I guess.

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