Include source in ProcessDefinitionEvent

[fjtirado]

Description

ProcessDefinitionEvent sent by runtimes when it starts up is missing the source. This forces data_index to query runtimes for the source.

Implementation ideas

After considering SWF operator to send the missing information into an additional event, it was decided to add the source information to the existing event.
This implies moving stuff from source addon to the core to make the source available to the event.
Once the information is included in the event, the source addon (with its endpoint) become optional. However, we will keep the addon for backward compatbility purposes.