Refactor macOS system DNS configuration (#2284)

On macOS use the recommended settings for providing split DNS. As per
the docs an empty string will force the configuration to be the default.
In order to to support split DNS an additional service config is added
for the local server and search domain settings.

see: https://developer.apple.com/documentation/devicemanagement/vpn/dns

client/internal/dns/host.go

@@ -15,6 +15,12 @@ type hostManager interface {
 	restoreUncleanShutdownDNS(storedDNSAddress *netip.Addr) error
 }
 
+type SystemDNSSettings struct {
+	Domains    []string
+	ServerIP   string
+	ServerPort int
+}
+
 type HostDNSConfig struct {
 	Domains    []DomainConfig `json:"domains"`
 	RouteAll   bool           `json:"routeAll"`