there is no way to register skin to a skeleton made by rendering server

### Tested versions

all godot 4 versions

### System information

win 11

### Issue description

i am trying to create a character using rendering server using mesh and skeleton data but i cant assign the the skin data wish result to character to deform incorrectly

### Steps to reproduce

there is no way to assign a skin
```gdscript
RenderingServer.skeleton_allocate_data(character.skeleton,skeleton.get_bone_count())
for bone:int in skeleton.get_bone_count():
RenderingServer.skeleton_bone_set_transform(character.skeleton,bone,trans*skeleton.get_bone_pose(bone))
for i:int in 8:
var instance:RID = RenderingServer.instance_create()
character.instances.append(instance)
RenderingServer.instance_set_transform(instance,trans)
RenderingServer.instance_set_scenario(instance,get_world_3d().scenario)
RenderingServer.instance_set_base(instance,meshs[i])
RenderingServer.instance_attach_skeleton(instance,character.skeleton)
```

### Minimal reproduction project (MRP)

.

Start with the RenderingServer calls shown in the issue, especially skeleton_allocate_data and instance_attach_skeleton, and investigate how mesh, skeleton, and skin data are represented in the Godot 4 rendering APIs. Reproduce the deformation problem on Windows with a minimal project; completion requires an agreed, working way to assign skin data or a clearly documented limitation.

Written by the indexing model from the issue text.