Add flatpak support for the desktop app version

Since we want a full fledged desktop application version for Adguard (especially for new users who know nothing about the terminal), would the devs mind adding the flatpak version of this application to flathub app store?

This issue is migrated from [here](https://github.com/AdguardTeam/AdguardForWindows/issues/5492#issuecomment-2959798524).

BTW, if you want to submit to flatpak , you can follow these guidelines as documented: [Flathub guide submission](https://docs.flathub.org/docs/for-app-authors/submission)