OCI: Break out the annotation generation code into a helper
OCI: Break out the annotation generation code into a helper

This doesn't change the behaviour, it just breaks out everything related
to flatpak-specific annotations into one place where it can be resued.

Closes: #2978
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak