Migrate azdev and azure-cli-diff-tool to pyproject.toml

[aryasadeghi1]

Related issue

Azure/azure-cli-extensions#7740, Phase 3 (modernise packaging to pyproject.toml).

What this does

• Migrate azdev and azure-cli-diff-tool to PEP 621 pyproject.toml with the setuptools.build_meta backend. Both setup.py files are removed.
• CI builds with python -m build; tox validates metadata with twine check instead of setup.py check -r -s.
• Add azdev/utilities/packaging.py, which resolves a package's build configuration from either setup.py or pyproject.toml. setup.py keeps precedence, and tool-only pyproject.toml files are not treated as distributions.
• Drop the setuptools<80 cap. It guarded setup.py develop, which azdev stopped calling in 0.2.12. The >=78.1.1 floor for CVE-2025-47273 is retained.
• Bump to 0.2.14b1.

Not included

azdev extension add|list|build still hardcode setup.py, so pyproject-only extensions are not yet supported. That is the next stage and is intentionally out of scope.

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This checklist is used to make sure that common guidelines for a pull request are followed.

• pylint azdev --rcfile=.pylintrc -r n

• flake8 --statistics --append-config=.flake8 azdev

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