chore(models): streamline all server models IPs (#942)

- Use `IPs []net.IP` for all server models
- Use `ips` JSON field for all server models
- Merge IPv4 and IPv6 addresses together for Mullvad

internal/provider/nordvpn/connection.go

@@ -21,15 +21,17 @@ func (n *Nordvpn) GetConnection(selection settings.ServerSelection) (
 		return connection, err
 	}
 
-	connections := make([]models.Connection, len(servers))
-	for i := range servers {
-		connection := models.Connection{
-			Type:     selection.VPN,
-			IP:       servers[i].IP,
-			Port:     port,
-			Protocol: protocol,
+	connections := make([]models.Connection, 0, len(servers))
+	for _, server := range servers {
+		for _, ip := range server.IPs {
+			connection := models.Connection{
+				Type:     selection.VPN,
+				IP:       ip,
+				Port:     port,
+				Protocol: protocol,
+			}
+			connections = append(connections, connection)
 		}
-		connections[i] = connection
 	}
 
 	return utils.PickConnection(connections, selection, n.randSource)