32 bits compatible extensions

Some applications require 32 bits compatible extensions to run binaries and build 32 bits projects.

E.g. [Bottles](https://github.com/bottlesdevs/Bottles/blob/177beca6ece040a8e13a6551de035ae799f9edee/com.usebottles.bottles.json#L26) use the `org.gnome.Platform.Compat.i386` extension which is not available in the elementary repository. We need that to be able to compile some libraries and run 32 bits binaries via wine.

FWIK the [org.gnome.Platform.Compat.i386](https://docs.flatpak.org/en/latest/available-runtimes.html#gnome) is based on top of the [org.freedesktop.Platform.Compat](https://docs.flatpak.org/en/latest/available-runtimes.html#freedesktop) extension, including the GNOME Platform. I don't know if the best solution is to mirror it in the elementary repositories or provide a variant (maybe it's already there?)

As a workaround I am getting the necessary libraries from the [AppImage](https://github.com/bottlesdevs/Bottles/blob/f51e7ce68393fd031d4c8008fb191641e331d0cc/com.usebottles.bottles.elementary.yml#L113) but the extension might be the best solution.

I am relatively new to the world of Flatpak, let me know if I'm wrong please 😁

_Originally posted by @mirkobrombin in https://github.com/elementary/flatpak-platform/discussions/61_