New extension classes to create a Model at the run-time

[rds1983]

This PR provides an extension classes to create Model at the run-time.
Usage is like this:

  // Create meshes
  var meshes = new List<ModelMeshDescEXT>();
  var mesh1 = new ModelMeshDescEXT();
  mesh1.Name = "mesh1";
  mesh1.AddModelMeshPart(indexBuffer1, 0, vertexBuffer1, 0, vertexBuffer1.VertexCount, primitivesCount1);
  meshes.Add(mesh1);

  var mesh2 = new ModelMeshDescEXT();
  mesh2.Name = "mesh2";
  mesh2.AddModelMeshPart(indexBuffer2, 0, vertexBuffer2, 0, vertexBuffer2.VertexCount, primitivesCount2);
  meshes.Add(mesh2);

  // Create bones
  var bones = new List<ModelBoneDescEXT>();
  var bone1 = new ModelBoneDescEXT();
  bone1.Name = "bone1";
  bones.Add(bone1);

  var bone2 = new ModelBoneDescEXT();
  bone2.Name = "bone2";
  bone2.Meshes.Add(mesh1);
  bone2.Meshes.Add(mesh2);
  bone1.Children.Add(bone2);
  bones.Add(bone2);

  var model = ModelBuilderEXT.Create(device, meshes, bones, bones[0]);
  ... 

Such API is enough to, say, load gltf at the run-time.