Obtain the correct list of available IP adresses in a delegated subnet via `az network vnet subnet list-available-ips`.

**Related command**
`az network vnet subnet list-available-ips`

**Is your feature request related to a problem? Please describe.**
We are currently unable to obtain the available amount of ips for delegated subnets, since it will **always** show the max amount of IPs as available for a delegated subnet in the portal and via the az cli command it also seems to return a list of ips even when the subnet is actually full.

**Describe the solution you'd like**
When running the az cli command it should return a list with **actual** available ips in a delegated subnet, this is currently not the case.

**Describe alternatives you've considered**
I don't believe there is anyway from the client side to obtain this information from the Azure API currently. I have opened a case before but they recommend we open a feature request, so here we are.