[Suggestion] Allow setting app label in assemble manifest file

Is your feature request related to a problem? Please describe. There is an exported_bins_path flag for binaries but no way to declaratively change the exported apps label.

Describe the solution you'd like I would like something like exported_apps_label that maps to distrobox-export --export-label

Describe alternatives you've considered I tried writing a bash script to export a list of apps as an init hook but distrobox-export can't run as root