resolves #1827

During the (long) investigation into the issue above we have found that in some cases environment variable reads can be very expensive in App Service. But we cannot fully cache the variables because during certain App Service events (slot swaps, for example), the environment variables can change out from underneath us.

This change adds a background monitor to watch for the changes in these few environment variables, then triggers an Options change when it occurs. This keeps the expensive reads off the hot path.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
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• I have added all required tests (Unit tests, E2E tests)