Ship metainfo.xml file with native packages

In Briar GTK I shipped a metainfo.xml file that is used by, e.g., GNOME Software to render a beautiful store entry with descriptions and screenshots (all translatable!).

I have no idea how to ship this file with Compose, nor how we would generate the version including all translations. At Briar GTK the build system Meson was doing this for us, therefore it has the appendix .in.