Support subsets in the storefront info

[jameswestman]

Flatpak allows us to add commits to a subset, which allows clients to broadly filter the repository: https://github.com/flatpak/flatpak/pull/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.

[razzeee]

I think this was done?