Add pulp-label serializer helpers to plugin api

[gerrod3]

I need this for pulp/pulp_container#2172

Assisted by: claude-4.5-sonnet

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough

[patchback]

Backport to 3.49: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 9b0aa39 on top of patchback/backports/3.49/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264

Backporting merged PR #7264 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/pulpcore.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/3.49/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264 upstream/3.49

4. Now, cherry-pick PR Add pulp-label serializer helpers to plugin api #7264 contents into that branch:

   $ git cherry-pick -x 9b0aa39f328664a3e42df97941e5f1a128ba2f42

   If it'll yell at you with something like fatal: Commit 9b0aa39f328664a3e42df97941e5f1a128ba2f42 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 9b0aa39f328664a3e42df97941e5f1a128ba2f42

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add pulp-label serializer helpers to plugin api #7264 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.49/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264

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 3.63: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 9b0aa39 on top of patchback/backports/3.63/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264

Backporting merged PR #7264 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/pulpcore.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/3.63/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264 upstream/3.63

4. Now, cherry-pick PR Add pulp-label serializer helpers to plugin api #7264 contents into that branch:

   $ git cherry-pick -x 9b0aa39f328664a3e42df97941e5f1a128ba2f42

   If it'll yell at you with something like fatal: Commit 9b0aa39f328664a3e42df97941e5f1a128ba2f42 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 9b0aa39f328664a3e42df97941e5f1a128ba2f42

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add pulp-label serializer helpers to plugin api #7264 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.63/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264

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 3.73: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.73/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264

Backported as #7268

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

[patchback]

Backport to 3.85: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.85/9b0aa39f328664a3e42df97941e5f1a128ba2f42/pr-7264

Backported as #7269

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