[AfO Landing Zones] Add AZ support for secured vhub

[victorar]

In the AfO Landing Zones reference implementation, add support for availability zones when AZFW is being deployed in a VWAN VHUB.

https://azure.microsoft.com/en-us/blog/boost-your-network-security-with-new-updates-to-azure-firewall/

[marvinbuss]

Hi @victorar,
Were you able to configure this successfully? I just tried to set this up manually, but the option is always greyed out in my case.
Do you know under which conditions this can be enabled?

I tested this in different regions: eastus, southcentralus and westeurope.