core: support onboard computer reboot/shutdown via MAVLink

### Current behaviour

We have the Commander API, but a MAVLink user should ideally not need to use specific BlueOS APIs for normal control of their vehicle.

Relevant to bluerobotics/cockpit#674

### Expected or desired behaviour

Support [`MAV_CMD_PREFLIGHT_REBOOT_SHUTDOWN`](https://mavlink.io/en/messages/common.html#MAV_CMD_PREFLIGHT_REBOOT_SHUTDOWN) usage for shutting down the BlueOS computer. This will presumably require some kind of lightweight monitoring of the incoming MAVLink stream(s) to check for the message (not sure if this should be handled by the existing MAVLink routing service, or MAVLink2REST, or something independent that's connected to one of those).

### Prerequisites

- [X] I have checked to make sure that a similar request has not already been filed or fixed.