Support syncing of DEP11 files

Is your feature request related to a problem? Please describe.
Add the syncing of DEP11 metadata files to enable the use of app stores based on Appstream like KDE Discover. This feature was mentioned / requested in #419 already. I am creating this separate issue for separate tracking of this request.

Describe the solution you'd like
When syncing a Repo containing DEP11 metadata files, those files should be synced as well.

Describe alternatives you've considered
There is no other alternative other than "not implementing" i can think of.

Additional context
As the files related to DEP11 aren't big and there is not many of them, i don't think a specific flag to toggle sync of those files is necessary. It should still be noted that there is a case where no DEP11 is available at all, this needs to be checked and the sync skipped accordingly.

Start by tracing the repository synchronization entry point and the existing handling of repository metadata files. Add coverage for syncing DEP11 metadata when present and for skipping it when absent; done means supported DEP11 files are synchronized without requiring a flag.