Gazebo: Add model for Sawppy rover

[srmainwaring]

This PR is for a version of Roger Chen's Sawppy Rover controlled using ArduPilot with a custom mixer scripted in Lua.

Details

• This is a port of the models from srmainwaring/curio and srmainwaring/ardupilot_gazebo_models to ArduPilot. The original version runs on ROS Melodic using a customised version of the ros_control package. This version is independent of ROS and the model is defined completely using Gazebos SDF markup.
• A detailed user guide is included in SITL_Models/Gazebo/docs/Swappy.md.
• Supports in-place / pivot turns (no skid - drives are differential and steered).
• This is an initial version of the model with fully functioning drive and steering but a restricted suspension system and a simple texturing. The plan is to add improvements incrementally with follow up PRs.

Context

• Adding the model and script to SITL_Models is discussed here: ESA ExoMy STEM Mars Rover.

Future Dev

The rocker-bogie suspension can be enabled once the support for closed kinematic chains available in Gazebo classic has been completed in Gazebo Sim. See the following for details:

• https://github.com/gazebosim/gz-physics/issues/25
• https://github.com/gazebosim/gz-physics/pull/352

[rmackay9]

Very nice to see the custom support. Just to link things together here is the related issue https://github.com/ArduPilot/ardupilot/issues/20563

[srmainwaring]

@rmackay9 I'll merge this PR to provide a base level example for mars-type rovers.

As a follow up, it might be useful to move the mixer script to libraries/AP_Scripting/applets? There are a few additions to make it stand alone which I'll outline in https://github.com/ArduPilot/ardupilot/issues/20563.