Currently if a user of libflatpak wants to list the related refs (such
as extensions and plugins) of something, they have three options:
1. They can parse the metadata manually with e.g.
flatpak_remote_ref_get_metadata() and then key-file operations, but
this means re-implementing parts of libflatpak and using key file
strings that are not actually public (FLATPAK_METADATA_KEY_...).
2. They can use flatpak_installation_list_installed_related_refs_sync()
but this only works for installed related refs not remote ones.
3. They can use flatpak_installation_list_remote_related_refs_sync() but
this lists all remotely available related refs, including ones that
may not be compatible with the installed version of the main ref
(because they don't match any of the values in the "versions"
metadata key).
So since none of these provide a way to get the remote related refs
corresponding to an installed application, add new API for that. For the
motivation of this see
https://gitlab.gnome.org/GNOME/gnome-software/-/issues/1132
2a111e5042