Support subsets in the storefront info #78
Description
Flatpak allows us to add commits to a subset, which allows clients to broadly filter the repository: flatpak/flatpak#3955. For example, in Flathub, we want to have a "FOSS-only" repository--it's the same OSTree repo, but a separate summary file that only contains commits marked as free software.
I'd like to implement this in a generic way by adding a subsets array field to the storefront info. flat-manager will add the app's commits to all of the given subsets.