[Feature request]: tell users if extra-data is being used to install/update applications through the Flatpak CLI

[rmnscnce]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Suggestion

Some apps on Flathub (e.g. Spotify, Sublime Text, and Zoom) use extra-data to package the app as Flatpaks. These Flatpaks do not distribute all of the required files through the Flathub remote, but rather the users download some of the files at install time and then a script is executed to apply those files to the Flatpak installation.

In my opinion, users should be let known that their apps require data that are distributed separately from the Flatpak remote(s) they have added. Moreover, OSTree delta updates are also inapplicable for extra-data files.