[Doc] Mention defaultProps relation to getSubLayerProps in LayerExtension documentation

[richard3d]

Link

https://deck.gl/docs/developer-guide/custom-layers/layer-extensions#getsublayerprops

Description

I had written an extension and noticed none of the props were getting passed down to the sub layers in my composite layer I applied the extension to. After looking at the docs I noticed getSubLayerProps, however the thing that made it confusing was that when I looked at other extensions such as DataFilterExtenson this method was not defined. Yet, its properties were being added to the composite sublayers when I tested it. Really all a user needs to do is define default props and the base getSubLayerProps in the LayerExtension interface will work just fine without writing a bunch of copy code.