Determine list of web apps to be included as available with Software

I am working on bringing back support for web apps into Software, with a focus on Progressive Web Apps, but other web apps will show up when installed via a browser (Epiphany/Chromium). The goal is to help users find and use a wider array of quality apps on Linux, and in order to facilitate that discovery Software needs to know about a set of PWAs that it can display as installable without any user configuration needed. I can work on a tool to produce AppStream component data from a set of URLs using the metadata found in web app manifests, and we can include that AppStream data with Software, with an option to disable it at build time in case some distributions don't want it.

Since this is a GNOME project we should only include PWAs which are open source.

cc @aday