Add Mesh.create_convex_shapes() to expose convex decomposition for scripts

[smix8]

Adds Mesh.create_convex_shapes() functions so scripts can make use of the convex decomposition with MeshConvexDecompositionSettings parameters.

Usage example:

var torus: TorusMesh = TorusMesh.new()
var settings: MeshConvexDecompositionSettings = MeshConvexDecompositionSettings.new()
settings.max_convex_hulls = 10
settings.max_concavity = 0.001
var convex_shapes: Array = torus.create_convex_shapes(settings)
print(convex_shapes)

In the past due to all the other engine functions using types that do not work with script bindings the only function exposed for scripting that could work with the MeshConvexDecompositionSettings parameters was the MeshInstance3D.create_multiple_convex_collisions(settings) function changed/added by #72152.

That MeshInstance3D function always had the down-side of being an extra node and also creating new StaticBody3D and CollisionShape3D nodes for compatibility instead of just the collision shapes. This caused a number of frequent complains from users that needed convex decomposition of meshes for procedural stuff but were only interested in the resulting shapes, not all the performance costing unneeded extras.