feat(ntp): enhances time sync with DHCP NTP and custom servers (#625)

* Ensure the mDNS mode is set every time network state changes

Eliminates (mostly) duplicate code

* Add custom NTP and HTTP time sync servers

Since the ordering may have been previously defaulted and saved as "ntp,http", but that was being ignored and fallback-defaults were being used, in Ordering, `ntp` means use the fallback NTP servers, and `http` means use the fallback HTTP URLs. Thus `ntp_user_provided` and `http_user_provided` are the user specified static lists.

* Add support for using DHCP-provided NTP server

internal/timesync/metrics.go

@@ -73,6 +73,7 @@ var (
 		},
 		[]string{"url"},
 	)
+
 	metricNtpServerInfo = promauto.NewGaugeVec(
 		prometheus.GaugeOpts{
 			Name: "jetkvm_timesync_ntp_server_info",