Implement split-attribute handling for `save_mesh()` and `load_meshes()`

[trexfeathers]

Implement split-attribute handling for save_mesh() and load_meshes() (load_mesh() should take care of itself). Might need to modify the Warnings from #7109

Assumption is that Iris save itself will already be handled thanks to the work in #7108. If not: make any necessary changes now

See parent issue for more:

• Allow global attributes on a Mesh #6085