[Snap] Implement NTP API

[benfrancis]

Implement a mechanism by which the gateway can check the status of an NTP server and restart it.

This is used by the gateway to check whether the clock has been set from the internet, in order to warn the user about potential certificate validation problems if the system clock is wrong.

E.g. https://github.com/WebThingsIO/gateway/blob/master/src/platforms/linux-raspbian.ts#L976