Add OMI_physics_gravity implementation and test files

[aaronfranke]

This PR adds a Godot implementation of OMI_physics_gravity.

This requires Godot 4.3 or later, plus several unmerged changes:

• Requires Godot PR 82878 Area2D/3D Gravity: Replace point bool with a type enum, add Target gravity type godotengine/godot#82878.
• Requires Godot PR 90230 Separate Shape3D resource logic in GLTFPhysicsShape godotengine/godot#90230.
• Exporting global gravity requires Godot PR 90225 GLTF export: Propagate property_list_changed from extensions godotengine/godot#90225.
• The example files were exported with Godot PR 88301 Add support for explicitly-defined compound triggers in GLTF files godotengine/godot#88301, but this is not required to run them.
• This PR contains custom code for local directional gravity, working around the lack of Godot PR 90166 Fix Area2D/3D not using node rotation for directional gravity godotengine/godot#90166 using Godot PR 82878.
• This PR expects that OMI PR 216 OMI_physics_gravity gltf-extensions#216 will be accepted.

To try it out, open either of the examples/omi_physics_gravity/test/test_*.tscn files and run the scene.