Add CollectionAnnotationsAttribute
This is used for holding IMAP ANNOTATION data, and exists in two basically identical copies right now, in pimcommon and kdepim-runtime, so move this here. Both copies had completely distinct sets of tests, so merge those.
@mlaurent Some of the test code from you was GPL-2.0-or-later, ok if we unify this to LGPL-2.0-or-later to match the rest here?
Set to draft as this needs to be integrated in one go with all other MRs adapting to this, so we don't end up with clashing symbols or duplicate attribute registrations.