Azure Public and Private DNS Zone Import and Export

## Description of the new feature

Add equivalent PowerShell cmdlets as the Azure CLI that allow you to:
* Export public and private DNS Zones to a bind compatible format
* Import public and private DNZ Zones from the export

The Azure CLI functionality is described here https://docs.microsoft.com/en-us/azure/dns/dns-import-export and the command reference is here: https://docs.microsoft.com/en-us/cli/azure/network/dns/zone?view=azure-cli-latest

At present there is no feature that allows you to backup DNS zones to protect against accidental deletion. While we could use resource locks, the problem with resource locks is that it prevents legitimate operations (e.g. you want to delete an individual record). While we can use the Azure CLI, we ideally want an automated solution that we can run from Azure Automation but you cannot currently run Azure CLI in Azure Automation. For this reason it would be great if equivalent PowerShell cmdlets were added.

Start with the Azure DNS import/export documentation and the Azure CLI zone command reference linked in the issue, then inspect how this repository exposes PowerShell cmdlets for Azure DNS. Done means equivalent commands can export public and private zones in BIND-compatible format and import those exports, with behavior suitable for Azure Automation.

Written by the indexing model from the issue text.