Added WAF rules for App Gateway and FrontDoor

[fbinotto]

PR Summary

Added new rules for App Gateway WAF and FrontDoor WAF.

Fixes #1470

PR Checklist

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

[fbinotto]

Thanks @BernieWhite, the issue was indeed with the missing type. I have now included the test for FrontDoorWAF.