[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