[balenaOS] Implement restart API

[benfrancis]

As a front end developer I want to be able to restart the gateway using an HTTP-based API.

This is currently possible on the Raspbian image, but not on balenaOS.

[benfrancis]

I'm not sure how feasible or necessary this is on balenaOS, but worth exploring.

I think on Raspbian this is currently mainly used to reboot the gateway after an update.

[benfrancis]

This appears to be exposed through a top-level action on the gateway via a controller, but I can't find anywhere that currently calls it from the front end or back end, so whilst potentially useful we probably do not need to block the release on this since balenaCloud can be used to restart services or the OS anyway.