Add support for (reading) zstd metadata

[dralley]

closes #3254

[dralley]

Filed as "misc" because we may need to backport it at some point, which we can't do as easily with "features".

edit: this is on hold. createrepo_c 1.0 is busted, it crashes with segfaults.

[dralley]

Should be unblocked soon: rpm-software-management/createrepo_c#385 (comment)

[dralley]

I'm going to add a test fixture with zstd compressed metadata and make sure we have CI for this, also.

[patchback]

Backport to 3.17: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply df9f512 on top of patchback/backports/3.17/df9f512f7bdff483a68af611cb33d32d21984f62/pr-3255

Backporting merged PR #3255 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pulp/pulp_rpm.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.17/df9f512f7bdff483a68af611cb33d32d21984f62/pr-3255 upstream/3.17

4. Now, cherry-pick PR Add support for (reading) zstd metadata #3255 contents into that branch:

   $ git cherry-pick -x df9f512f7bdff483a68af611cb33d32d21984f62

   If it'll yell at you with something like fatal: Commit df9f512f7bdff483a68af611cb33d32d21984f62 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x df9f512f7bdff483a68af611cb33d32d21984f62

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add support for (reading) zstd metadata #3255 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.17/df9f512f7bdff483a68af611cb33d32d21984f62/pr-3255

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.18: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.18/df9f512f7bdff483a68af611cb33d32d21984f62/pr-3255

Backported as #3272

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.19: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.19/df9f512f7bdff483a68af611cb33d32d21984f62/pr-3255

Backported as #3273

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.22: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.22/df9f512f7bdff483a68af611cb33d32d21984f62/pr-3255

Backported as #3274

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.