AVD Add-On: Fix Host Pool Registration

[jamasten]

API Version 2019-12-10-preview for host pools has been updated to prevent the output of the host pool registration token. Update the host pool registration to use the DSC extension instead.

[jamasten]

I attempted to use the DSC extension. However, the storage account used by the PG does not allow enumeration of the blobs so finding the current ZIP file is still mystery. Instead, I kept the current deployment using the custom script extension with the addition of a new key vault and custom script extension where the token is pulled using API calls and stored as a key vault secret.

[jamasten]

The PG released a list function to get the token and since yesterday, 8/27/24, has landed in the US sovereign clouds.