add
  open fun addPersistentStyleCustomLayer(layerId: String, layerHost: CustomLayerHost, layerPosition: LayerPosition?): Expected<String, None>
Adds a new style custom layer.
Whenever a new style is being parsed and currently used style has persistent layers, an engine will try to do following:
- keep the persistent layer at its relative position 
- keep the source used by a persistent layer 
- keep images added through addStyleImage method 
In cases when a new style has the same layer, source or image resource, style's resources would be used instead and MapLoadingError event will be emitted.
Return
A string describing an error if the operation was not successful, or empty otherwise.
Parameters
layer
A style layer identifier.
layer
The CustomLayerHost.
layer
If not empty, the new layer will be positioned according to LayerPosition parameters.