Support syncing of DEP11 files

[DEiselt]

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.