core: frontend: vehicle-setup: allow setting water density

### Current behaviour

We currently display the `BARO_SPEC_GRAV` parameter value for external barometers on water-based vehicle, but we don't actually allow the user to change it, so even if they realise they're using a freshwater assumption for depth estimation while operating in seawater, it's not obvious how to change that.

### Expected or desired behaviour

For vehicles that use it, add an editor for that parameter on the Baro calibration page, and potentially also on the overview page.

Ideally this would be via a specifier of [salinity](https://en.wikipedia.org/wiki/Salinity#/media/File:Water_salinity_diagram.png) instead, and calculate a more realistic depth estimate that factors in the temperature and salinity together (instead of just directly defining a static density value), but a proper integration of that would require modifying the autopilot firmware too, which is out of scope for BlueOS.

### Prerequisites

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