Add support to sync DEP11 metadata files

[DEiselt]

This change adds optional support for syncing of DEP11 metadata files, enabling the use of application stores like KDE Discover which are based on Appstream.

I reworked PR #1277 for the current main because the it was developed on basis of version 3.5, which caused a lot of conflicts trying to rebase.

I mark this as draft, because i'm not sure how to handle the feature toggle correctly and would like some input. As of right now, the toggle only affects the syncing part. This in turn means that if a repo is syncing with DEP11 sync enabled and the flag is disable afterwards, the synced DEP11 files are still present and published.

My question would be how this should be handled correctly. Options i see are:

• check the feature toggle both on sync and publish -> leaves unused files as of the above example
• clean up DEP11 files somehow, maybe when disabling the feature?
• keep those files and still publish them, which might cause outdated metadata to be published

I'll also re-evaluate how to write tests for this feature but @hstct's comment from the last PR still stands. I just want to get the review / feedback rolling for now.