feat: Add flatpak support

[noelmiller]

Purpose

This PR is to add installation of flatpaks as part of the ISO build process. This is something that Fedora does for Silverblue and Kinoite, so we are utilizing the templates they use to build.

Things that are working

• Container builds properly and is usable
  Example:

docker run --rm --privileged --volume .:/isogenerator/output -e FLATPAK_REMOTE_REFS="app/org.videolan.VLC/x86_64/stable runtime/org.kde.Platform/x86_64/5.15-23.08" -e SECURE_BOOT_KEY_URL=https://github.com/ublue-os/akmods/raw/main/certs/public_key.der -e ENROLLMENT_PASSWORD=ublue-os -e IMAGE_NAME=silverblue-main -e IMAGE_TAG=latest --pull always ghcr.io/ublue-os/isogenerator:pr-56

This will successfully install VLC

Current issues with this PR

• Origin is set to fedora which breaks updates for flatpaks installed from flathub (hopefully be fixed in: Move hardcoded flatpak remote to configuration rhinstaller/anaconda#5493)

  • We will need to edit anaconda.conf and build a copr of anaconda with this specific patch in it.
    

• Action is broken, needs to be refactored to use the container