Add central sleep patching fixtures

[mccoyp]

Description

Iterates on #35670. Since automatically applying these patches universally is problematic for some test suites, this PR instead makes the fixtures available for easy use by anyone who needs them. Documentation is also updated to explain their purpose and show how to include them in the proxy startup fixtures that most tests already use.

This has been locally tested to confirm that the change is compatible with libraries that do and don't use azure-sdk-tools.

Storage CI run: https://dev.azure.com/azure-sdk/public/_build/results?buildId=3796135&view=results
Core CI run: https://dev.azure.com/azure-sdk/public/_build/results?buildId=3796137&view=results

cc @scbedd

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[scbedd]

This is so cool! Thanks McCoy!

[mccoyp]

/check-enforcer override