Allow clearing PhysX mesh cache or creating convex meshes from numpy arrays

[ZoneLikeWonderland]

Is your feature request related to a problem? Please describe. Yes. PhysxCollisionShapeConvexMesh::LoadMultiple uses MeshManager to cache PhysX meshes by filename and never invalidates them (see src/physx/collision_shape.cpp and src/physx/mesh_manager.cpp).

In a long-running process this causes:

• Modified meshes with the same filename cannot be reloaded (old cooked mesh is always reused).
• Memory usage keeps growing as more mesh files are loaded and only drops when the engine/process exits.

Describe the solution you'd like Either:

• an API to clear or disable the PhysX mesh cache (C++ and/or Python, e.g. engine.clear_mesh_cache()), or
• an API to build convex collision shapes directly from numpy vertices / faces arrays, bypassing filename-based caching.

Describe alternatives you've considered

• Changing the filename each time the mesh changes (works around reload, but makes memory growth even worse).
• Restarting the whole process/engine to release cached meshes.

Both are impractical for long-running services or RL training.

Additional context Python call site: sapien.physx.PhysxCollisionShapeConvexMesh.load_multiple(...).