Add extension point for utilities

[gasinvein]

This would allow building utilities like protontricks as flatpak extensions.
Influenced by #505

[flathubbot]

Started test build 10906

[flathubbot]

Build 10906 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/10387/com.valvesoftware.Steam.flatpakref

[nanonyme]

Why did you remove the git repo?

[gasinvein]

@nanonyme FD.o gitlab is down at the moment. I can't build this locally otherwise.

[nanonyme]

Wow, that's really lame.