Expose physics RID

[Ryan-000]

It's quite useful when you need to interact with the terrain from PhysicsDirectSpaceState3D.

[TokisanGames]

The function call will look like terrain->get_rid(), but this is not the RID of Terrain3D. Call it get_collision_rid().

If debug collision is enabled, and _debug_static_body != nullptr, you need to return _debug_static_body->get_rid() or RID() if nothing.

Bear in mind #278 will redesign collision and the API might change in the future.

[TokisanGames]

Thanks for the change. I have updated this for you so we can merge it in.

You made your change on the main branch instead of another as instructed in the Godot pull request workflow. I had to force push into your main branch, which has probably broken it on your system. That means you might not be able to pull changes. If so, you'll have to delete your local main branch and download it again, or do another clone. In the future, always make another branch to make changes in.