Previously the code assumed that appstream data was stored in a separate
OCI image in the registry. Replace that with storing the appstream data
and icons as image annotations. When we download a new version of the
image index, the appstream data is combined, and icons are downloaded
as necessary.
Since there is no longer a content hash for the appstream data, it's
not practical for the user to download the appstream data and pass it
to the system helper, instead the system helper just downloads the
appstream data directly.
Closes: #1910
Approved by: alexlarsson
baa7d35ca1