Check for _CountingAttr subclasses

[LecrisUT]

Summary

The main purpose for this is to allow custom fields to extend the field decorators. This does technically enable that by subclassing the private class and adding the appropriate methods as in #1543.

I do not believe this is the best approach for handling #1543, it's just the quickest way to unblock it. The class should probably become something more public. If there are other ideas happy to explore them.

Pull Request Check List

• I acknowledge this project's AI policy.
• This pull requests is not from my main branch.
  • Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.
• There's tests for all new and changed code.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file typing-examples/baseline.py or, if necessary, typing-examples/mypy.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• The documentation has been updated.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signatures of @attr.s() and @attrs.define() have to be added by hand too.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
    The next version is the second number in the current release + 1.
    The first number represents the current year.
    So if the current version on PyPI is 26.2.0, the next version is gonna be 26.3.0.
    If the next version is the first in the new year, it'll be 27.1.0.
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• Changes have news fragments in changelog.d.