Add appstream support

[PhilipCramer]

Fixes # N/A - New feture

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Also, to speed up things, if you could kindly "Allow edits and access to secrets by maintainers" in the
PR settings, as this allows us to rebase the PR on master, fix conflicts, run coverage and help with
implementing code and tests.

Description of the Change

Adds option to include AppStream (DEP-11) metadata, so software centers like GNOME Software and KDE Discover work against aptly-published repos.

The feature is optinal with a --with-appstream flag to mirror create/edit (and the corresponding API field) that downloads and republishes dep11 files, same pattern as --with-sources and --with-installer.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

❌ Patch coverage is 82.45614% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.71%. Comparing base (9defe70) to head (db4668c).

Files with missing lines	Patch %	Lines
deb/remote.go	87.75%	3 Missing and 3 partials ⚠️
api/mirror.go	16.66%	4 Missing and 1 partial ⚠️
deb/publish.go	73.68%	3 Missing and 2 partials ⚠️
cmd/mirror_edit.go	60.00%	1 Missing and 1 partial ⚠️
cmd/mirror_update.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1543      +/-   ##
==========================================
- Coverage   76.96%   76.71%   -0.26%     
==========================================
  Files         160      160              
  Lines       14757    14861     +104     
==========================================
+ Hits        11358    11400      +42     
- Misses       2266     2326      +60     
- Partials     1133     1135       +2     

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