Make the "Add existing variation" widget pluggable

Heh, here I am 10+ years later. Found this while searching, forgot I had been here. This is partly being solved at ✨ Allow select widget for "Add existing items" Needs work .

Yah, we have a lot of zombies down the well.
If this is still relevant (otherwise close) this goes to 3.x as main development branch.

Supporting Media Library would be awesome.