Azure Firewall : Not return DNS setting with Get-AzFirewall

## Description of the new feature
Return DNS setting with Get-AzFirewall

Currently, Get-AzFirewall command does not return the DNS setting, so executing the following commands:

```
$azfw = Get-AzFirewall -Name "firewall name" -ResourceGroupName "resouce group name" -debug
```
```
$azfw = Get-AzFirewall -Name "firewall name" -ResourceGroupName "resouce group name"
$azfw | Format-List -Property *
```
```
$azfw = Get-AzFirewall -Name "firewall name" -ResourceGroupName "resouce group name"
$azfw.DNSServer
```

## Proposed implementation details (optional)
Return the following result when executing Get-AzFIrewall command:

```
"additionalProperties": {
"Network.DNS.EnableProxy": "false",
"Network.DNS.Servers": "8.8.8.8"
},
```