Modify DNS Search Domain

It seems that Netavark starts aardvark with the default DNS filter (`.dns.podman`). This should be customizable:

```
$ dig +short @10.88.0.1 container.podman.mycustomdomain
10.88.0.12
```

Aardvark sets the filter domain here:

https://github.com/containers/aardvark-dns/blob/8377c0a98bff609626cff273f829c34eca39ab08/src/main.rs#L69

This means that Netavark would need to set the filter domain here:

https://github.com/containers/netavark/blob/8377c0a98bff609626cff273f829c34eca39ab08/src/dns/aardvark.rs#L93

An optional entry in the network configuration that defaults to `dns.podman` would suffice here, much like this code:

https://github.com/containers/netavark/blob/3a1902a0dbc98812476e6846b87b52376da39c80/src/network/bridge.rs#L166-L168