Copy and Paste references between projects

There's a bunch of lacking features:

- [x] ~Support copying & pasting from Solution Explorer -> File Explorer via CF_HDROP (the other way already works)~ Moved to: https://github.com/dotnet/project-system/issues/5910.
- [x] ~Support copying & pasting between Visual Studio instances. We currently use CF_VSREFPROJECTITEMS/CF_VSSTGPROJECTITEMS to copy between the same instance, but this format isn't usable in another instance because it contains the result of IVsSolution.GetProjrefOfItem which is only exchangeable in the same solution. We'd need to come up with a new format that can populate IDiskBasedDragDropItem.~
- [ ] Support copying & pasting references of all kinds; project, binary, NuGet, etc. We'd need to copy with new format to represent this, unless we can get a ref of them via IVsSolution.GetProjrefOfItem.

In CPS there's two concepts;

- IPasteDataObjectProcessor - handles paste
- ICopyPackager - handles copy