Add Flatpak target and workflow

[gastmaier]

Generate a flatpak package[1], as we already do with Scopy.

Flatpak policy is set in the manifest files instead of through CMake, therefore during flatpak build:

• libiio is compiled with -DINSTALL_UDEV_RULE=OFF.
• CMakeLists.txt does not install org.adi.pkexec.osc.policy file.

The flatpak build was tested with org.gnome.Platform version x86_64/3.38 (oldest available in flathub) and x86_64/43 (latest).

The Flathub/Flatpak Builder GitHub Action is being used, which will pull an image from the author's account (gnome developer, flathub maintainer) (if that's not ok, we can create our own image and publish in Docker hub, like in the others workflows).

[1] The flatpak package is available for master and flatpak branches, at flatpak build -> Summary -> Artifacts, we also could include to the Release Assets list.

Suggestions are welcome.