Attitude Indicators: support both vehicle- and horizon-focused perspectives

### Problem to solve

Depending on perspective and intent, vehicle operators may find it more intuitive to have an external/third-person view on where the vehicle is relative to some target (e.g. how much it has rolled+pitched relative to the horizon), or an in-cockpit/first-person focus considering where the environment is around the vehicle (e.g. where the horizon/target is relative to the horizontal/forwards point of view).

### Desired approach

1. HUD:
- [ ] add a toggle-able follower display option, which shows the rear of the vehicle from the perspective of a following observer
- [ ] allow changing its size

2. Gauge:
- [ ] Add a vehicle-centric vs horizon-centric toggle-switch
- These effectively switch between perspectives of "what am I doing?" (fixed horizon, rotating/moving vehicle indicator) and "where am I going?" (fixed vehicle indicator, rotating/moving horizon)
- [ ] Add a "show vehicle rear" option to the horizon-centric view
- [ ] Add colour selection support for the sky and ground
- In some places/environments, a dark sky with light ground may be more intuitive

### Additional context

Forced switches between these options have [been the cause of various (crewed) aircraft crashes before](https://aviation.stackexchange.com/questions/51791/how-is-a-confusion-possible-between-western-and-russian-attitude-indicators), and it seems worthwhile for Cockpit to provide options rather than prescribing a particular convention.

This has also been [asked for in the BR forum](https://discuss.bluerobotics.com/t/leveling-horizon-in-cockpit-is-opposite-compared-to-rov-tilt/20194/11), and [is already supported in MissionPlanner](https://discuss.ardupilot.org/t/roll-inverted-arduplane-vtol/133342/5).

### Prerequisites

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

No files or tests are named. Start by locating the HUD and Gauge attitude-indicator widgets in the Cockpit frontend and review how their current vehicle and horizon displays are implemented. Done means the requested perspective, vehicle-rear, size, and sky/ground color options are available and usable in both widgets.

Written by the indexing model from the issue text.