Extend API for collision creation for meshes

[Shatur]

We currently have PendingConvexCollision to import a mesh and a generate collision for it. But it generates ConvexHull which means that it create convex polygon from the vertices. But it doesn't work well if model represented as a concave mesh.
For example, I have a house prototype model:

If I apply PendingConvexCollision for it, I won't be able to enter the building though the door because the collision is represented as a concave polygon. In Godot you have two options to workaround it:

1. Use convex decomposition. Rapier supports convex decomposition too, we could export the API.
2. Create a trimesh polygon from a mesh. Godot uses import hints for it. In 4.0 a nice import menu will be available (which also supports trimesh collision generation!). We could also implement this feature with API similar to PendingConvexCollision: user spawns a scene and adds PendingConvexCollision. This feature depends on Add Body::TriMesh definition #41.