[Storage] Refactor extensions packaging + other misc changes

[jalauzon-msft]

This change applies a few changes to azure-storage-extensions ahead of release:

• Adds the checksums module and convert azure.storage.extensions to purely namespace packages for future expansion.
• Remove Python 3.9 support
• Add typing information to extensions

[Copilot]

Pull request overview

This PR refactors the azure-storage-extensions package layout to introduce a checksums subpackage and updates Azure Storage clients to use the new import path and an ext-checksums extra for installing the native extensions ahead of release.

Changes:

• Add a new azure.storage.extensions.checksums package (including crc64 extension wiring) and update azure-storage-extensions packaging/build config (including dropping Python 3.9).
• Add ext-checksums extras to azure-storage-blob, azure-storage-file-share, and azure-storage-file-datalake to pull in azure-storage-extensions.
• Update shared validation helpers in storage libraries to import azure.storage.extensions.checksums and call checksums.crc64.compute.

Reviewed changes

Copilot reviewed 15 out of 17 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
sdk/storage/azure-storage-queue/azure/storage/queue/_shared/validation.py	Switch extension presence check and CRC64 calls to azure.storage.extensions.checksums.
sdk/storage/azure-storage-file-share/setup.py	Add ext-checksums extra to install azure-storage-extensions.
sdk/storage/azure-storage-file-share/azure/storage/fileshare/_shared/validation.py	Update extension import/check and CRC64 usage to checksums subpackage.
sdk/storage/azure-storage-file-datalake/setup.py	Add ext-checksums extra to install azure-storage-extensions.
sdk/storage/azure-storage-file-datalake/azure/storage/filedatalake/_shared/validation.py	Update extension import/check and CRC64 usage to checksums subpackage.
sdk/storage/azure-storage-extensions/tests/test_crc64.py	Update tests to import CRC64 via azure.storage.extensions.checksums.
sdk/storage/azure-storage-extensions/setup.py	Adjust packaging to ship checksums package and build the crc64 extension under it; raise minimum Python to 3.10.
sdk/storage/azure-storage-extensions/README.md	Update installation guidance to recommend installing via storage SDK extras and reflect Python 3.10+ support.
sdk/storage/azure-storage-extensions/pyproject.toml	Update cibuildwheel build targets to drop CPython 3.9.
sdk/storage/azure-storage-extensions/MANIFEST.in	Include native sources/headers and typing artifacts in source distributions.
sdk/storage/azure-storage-extensions/azure/storage/extensions/checksums/py.typed	Mark checksums as a typed package.
sdk/storage/azure-storage-extensions/azure/storage/extensions/checksums/crc64/helpers.h	Make constants/functions static in header to avoid multiple-definition/linkage issues.
sdk/storage/azure-storage-extensions/azure/storage/extensions/checksums/crc64/crc64module.c	Add the C extension module implementation under the new checksums/crc64 path.
sdk/storage/azure-storage-extensions/azure/storage/extensions/checksums/crc64.pyi	Provide typing stubs for the crc64 extension module.
sdk/storage/azure-storage-extensions/azure/storage/extensions/checksums/init.py	Expose crc64 from the checksums package.
sdk/storage/azure-storage-blob/setup.py	Add ext-checksums extra to install azure-storage-extensions.
sdk/storage/azure-storage-blob/azure/storage/blob/_shared/validation.py	Update extension import/check and CRC64 usage to checksums subpackage.