FEAT-#7401: Implement DataFrame/Series.attrs

[noloerino]

What do these changes do?

[STILL A DRAFT] Implement df.attrs by annotating relevant functions with a new @propagate_self_attrs annotation that automatically copies self._attrs if the result is detected to be a new DataFrame/Series.

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• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves BUG: Writes to DataFrame.attrs are not preserved #7401
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date