add extension/plugin support for Octave packages

See conversation with Octave maintainers here: https://savannah.gnu.org/bugs/index.php?60319

Original request to Octave maintainers:
"_Hi there. Description of the request: Add possibility to install the extra Octave functions and packages as Flatpak add-on, right from the Flatpak install page of Octave. Thank you for considering my request and keep up the good work_"

Start by reading the linked Octave maintainer conversation at https://savannah.gnu.org/bugs/index.php?60319 and inspect how this Flatpak application is packaged. Clarify which Octave functions or packages are in scope and how add-ons should appear on the Flatpak install page; done means the agreed packages can be installed through that flow.

Written by the indexing model from the issue text.