Gradle: Add flatpakGradleGenerator task

[vidplace7]

This PR adds a new gradle task flatpakGradleGenerator

run with:

./gradlew flatpakGradleGenerator --no-configuration-cache

this generates a json file flatpak-sources.json containing all dependencies needed to build :desktop.
When a flatpak builds using this flatpak-source.json file, all dependencies are stored to a dir offline-repository which gradle now leverages as a fallback maven repository.

See example flatpak-sources.json

In the future we may distribute this as part of GitHub Releases? (So they can be consumed by FlatPak builds directly).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 12.23%. Comparing base (6d132ed) to head (b3b2e97).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4807   +/-   ##
=======================================
  Coverage   12.23%   12.23%           
=======================================
  Files         530      530           
  Lines       17753    17753           
  Branches     2652     2652           
=======================================
  Hits         2172     2172           
  Misses      15267    15267           
  Partials      314      314           

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[jamesarich]

lgtm, we can roll once you're good with it @vidplace7 👍

[vidplace7]

This isn't quite including everything we need yet. Still WIP.