Get-AzApplicationGatewayBackendAddressPool option for Virtual Machine -BackendVMs ?

## Description of the new feature
It appears as though the only options for managing Gateway Backend pools is via FQDN or IP Address. Can there also be an option for Virtual Machine? I've found managing application gateways via PowerShell much more reliable than using the portal. I also have servers in an availability set so it would be nice to automate rolling updates.

## Proposed implementation details (optional)
Maybe the -BackendVM option can be set previously defining the NIC? like
$BackendVMNic = Get-AzVM -ResourceGroupName "ResourceGroup11" -Name "VirtualMachine07"
$BackendVM = $BackendVMNic.NetworkProfile

Or can default to the only NIC there if that applies?

Thanks!