[WorkFlow] Flathub distribution pipeline

We will publish the app on flathub as flatpak after testing the functionality on linux.

reference:
- https://docs.flathub.org/docs/for-app-authors/submission
- https://github.com/flathub/com.heroicgameslauncher.hgl
- https://docs.flatpak.org/en/latest/electron.html

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[Update] This does not require any changes in this repo, so avoid raising PRs for it