`Microsoft.Network/azureFirewalls ` - `privateIPAllocationMethod` property is not documented

Under the `ipConfigurations` array element, the `privateIPAllocationMethod` property is not documented, but it is returned on a GET, and possibly accepted on a PUT:
https://github.com/Azure/azure-rest-api-specs/blob/b2f2423182b461f9b556ff68e9db7e93085eda7f/specification/network/resource-manager/Microsoft.Network/stable/2021-02-01/azureFirewall.json#L391-L408

First reported here: https://github.com/Azure/bicep/issues/784#issuecomment-949981405

Start with specification/network/resource-manager/Microsoft.Network/stable/2021-02-01/azureFirewall.json at the referenced ipConfigurations schema. Check how privateIPAllocationMethod is represented in the GET response and whether the schema should accept it on PUT. Done means the property is documented in the Azure Firewall specification with the appropriate schema details.

Written by the indexing model from the issue text.