Allow other modules to extend the webform content creator functionality

The webform content creator is a great module and a real asset to the webform suite of modules, the issue I see is that at the moment field support is limited to just what the module provides where it feels like this could be easily opened up to make use of the plugin system.

I realised this as I was trying to add a patch to enable the mapped value to be adapted, in my case to allow address fields to be utilised.

I suspect the best approach would be to add a ContentCreatorSourceMap plugin, which would be selectable when mapping the field, with just a few attributes

* label
* [webform_element_types]

and would just have a mapValue() method.

By default you could just have an EntityReferenceContentCreatorSourceMap, CheckboxContentCreatorSourceMap, DefaultContentCreatorSourceMap which would just allow the logic in the entity mapNodeField method to be largely abstracted away. And could then be easily extended for other element types.

If I get time I'll try to take a bit more of a look at this, if it's something you think makes sense.