Add Extension Point

[p1u3o]

We could add an extension point for further archive/extract utilities. This could be more of a generic extension point so other archive utilities (I'm not sure how KDE's works?) could use them too.

e.g. rar so people can create rar archives (and cbrs). This isn't under a free license so may need to be extra-data and avoids having to pollute the main Flatpak with non-free software.

I don't mind making the MR for these if this is the route people agree on.