Add OMI_vehicle_* implementation and test files

[aaronfranke]

Work-in-progress implementation of omigroup/gltf-extensions#231

Test glTF files: https://github.com/aaronfranke/omi-godot/tree/omi_vehicle/examples/omi_vehicle/gltf

Test scenes: https://github.com/aaronfranke/omi-godot/tree/omi_vehicle/examples/omi_vehicle/test

Not required but nice to have: godotengine/godot#94862

[fire]

what is needed to test?

[aaronfranke]

@fire To test, switch to this PR's branch, open the project in Godot 4.3 stable, and run either res://examples/omi_vehicle/test/car_in_truck_town/car_in_truck_town.tscn or res://examples/omi_vehicle/test/space_station/space_station.tscn.

[yankscally]

looking good, I've tested out truck town demo (and also saw the demo a few times too).