Support configuration of hardware watchdog devices

There's currently no way to configure a hardware watchdog timer for Ubuntu Core other than using a custom solution to inject a configuration file into a factory image.

The only problem with this approach is that snapd doesn't have any awareness of this file, so updating and/or removing this file isn'nt easily handled.

It might make sense to add both RuntimeWatchdogSec and ShutdownWatchdogSec to the core snap's config hook. This would allow a gadget snap to initially configure these values. That said, I'm not sure this approach handles the use case of OTA-based updates to these values (ie. from a gadget snap update).

This is being reported as of snapd 2.23.1.