add the external raw data to the entity

Fixed in v3 to also allow the saving of source fields that were not mapped and therefore not provided by ::toRawData().
Two methods were added to access source raw data: setSourceRawData() and getSourceRawData().
The use of "Source" in the name is to avoid confusion with existing method toRawData().

Automatically closed - issue fixed for 2 weeks with no activity.