[Modules] Added support for Microsoft.web/sites/slots

[ladlavj]

Description

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Type of Change

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Update to documentation

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• My corresponding pipelines / checks run clean and green without any errors or warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (readme)
• I did format my code

[ladlavj]

@microsoft-github-policy-service agree

[AlexanderSehr]

Successfully tested myself. It works and I'll perform the only remaining change as it relates to the test setup exclusively (i.e., merge the slot with the webapp test). Thank you @ladlavj