Use identity-based authentication for App Configuration

[pazdedav]

PR Summary

New rule created to disable local authN (keys, SAS tokens) to Azure App Configuration and favor Azure AD authN instead:

• Azure.AppConfig.DisableLocalAuth.md docs page created
• New rule added to Azure.AppConfig.Rule.yaml
• New test section added to Azure.AppConfig.Tests.ps1
• Test template Resources.AppConfig.json updated, so one of the configStores passes, another fails

Fixes #1691

PR Checklist

• PR has a meaningful title
• Summarized changes
• Change is not breaking
• This PR is ready to merge and is not Work in Progress
• Rule changes
  • Unit tests created/ updated
  • Rule documentation created/ updated
  • Link to a filed issue
  • Change log has been updated with change under unreleased section
• Other code changes
  • Unit tests created/ updated
  • Link to a filed issue
  • Change log has been updated with change under unreleased section

[BernieWhite]

@pazdedav Awesome work! All good to merge.