Problem

Like #7927, ConnectionPatch is only defined for 2D coordinate systems. My use case is I've got a 2d plot that's a slice of a 3D plot and I wanna draw an arrow from the 2d image to the slice it corresponds to in the 3D view:

Proposed solution

It'd be nice if ConnectionPatch (and annotation) could take in 3D coordinates/3D axes and internally do the transforms internally. I figure implementing this issue should also more or less close #7927, mostly because I think the implementation approach should include factoring out the annotation coordinate system handling as described in #22223 (comment)