azure_firewall: Define a dedicated property for SNAT private ranges

Currently, when users want to specify SNA private ranges, one will set the special key `Network.SNAT.PrivateRanges` in [`additionalProperties`](https://github.com/Azure/azure-rest-api-specs/blob/be2510a84b5da51c9b83d1a9fddf148bb7f15ef3/specification/network/resource-manager/Microsoft.Network/stable/2020-03-01/azureFirewall.json#L553), with the value of ip ranges.

Is it possible to consolidate this into a dedicated property of its own, otherwise it is not that reliable to create toolings based on this special key, as it is possible to have breaking changes (e.g. renaming the key or change the format of the value)?

Even more, is there any document I can refer to which enumerates all the available KVs for the `additionalProperties`, so that I might come up with a PR to make `additionalProperties` a predefined model, rather than a flexible map?