Find a common way to build typed grid elements and system participants?

[sebastian-peter]

Once #773 is merged:

SystemParticipantSource.buildTypedSystemParticipantEntityData is used to build various typed system participants, while findAndAddType and addTypeToEntityData in EntitySource are used by RawGridSource.buildTypedConnectorEntityData to build typed grid assets.

They look somewhat similar and do a somewhat similar thing, so maybe one could abstract the functionality in a way that works for both use cases.

Originally mentioned by @sebastian-peter in #773 (comment)