Build Flatpak in CI

[AFCMS]

Fix #14652

To do

This PR is a Work in Progress.

• Add Flatpak manifest to repository (copied from Flathub with the following fixes)
  • Correct app ID (Rename app flathub/net.minetest.Minetest#5)
  • Enable Wayland support (as a default), only enable X11 as a fallback (since the next version will be SDL2)
  • Use local code (sadly using a git source type for the current directory instead of the dir type doesn't work in CI, which prevents from using CCACHE for the build for some reasons)
  • Add --share=ipc to permissions, apparently (?) it could affect X11 performances
• Add CI workflow to build the image
• Add the Flatpak bundle as CI artifact
• Add GH Attestations support for the bundle (allow to make sure a file comes from the CI workflow and link to the logs)
• Disable again attestations on pull requests
• Add some kind of documentation for the attestations?
• Put the manifest in another directory?
• More testing

On the used action side, this is a bit concerning (several warnings after run, much much lower performances in upload):

• Migration to upload-artifact version 4 flatpak/flatpak-github-actions#184

How to test

See workflow runs, read diff.

Try installing the bundle from the artifacts.

[Zughy]

Unfortunately, this feature is not on the roadmap and hasn't received a concept approval. Non-roadmap non-bugfix PRs have 30 days to be supported by a core dev, otherwise they should be closed. These PRs can also be pre-approved if they close an issue that is concept approved.