Rover/Boat: add support for water barometers

[ES-Alexander]

Feature request

Is your feature request related to a problem? Please describe.

1. It's difficult to tell how loaded a boat is, especially from telemetry
   • Multiple barometers at opposite ends of a boat (and/or in opposite hulls of a catamaran) could even potentially provide high-resolution pitch or roll estimates, to estimate load distribution (which impacts travel efficiency and controller tuning)
2. Boats have no feedback if they have water ingress or get overloaded
   • If a boat with a winch tries to pick up something heavy (or something lands on it) it could sink without warning
3. It's safety-critical for an underwater crawler to tell how deep it is, to prevent hull/component implosion
   • This isn't a boat, but is still Rover

Describe the solution you'd like

1. BARO_TYPE_WATER sensors should be supported in Rover (at minimum in the boat firmware)
   • I believe this would at least require specific gravity parameter support, for the water density
   • I'm unsure what else is required - the rest of the driver code might already be compiled in?
2. A pressure/altitude failsafe should be implemented
   • Should ideally share implementation with the proposed Sub one
3. Ideally barometer positions could be specified, and used as an alternative roll/pitch source (or at least in a warning/failsafe/pre-arm-check capacity if the average difference gets/is too high)

Describe alternatives you've considered
A leak failsafe could at least warn about active water ingress, but that's somewhat independent functionality.

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[X] Rover
[ ] Submarine

Additional context
ArduSub already supports water barometers, so could be used for inspiration.