dataservice should support related table field

@OriHoch:

> maybe it's worth to create a new class for this type of fields
something like:
("committee", KnessetdataServiceRelatedField("CommitteeID", dataservice_class=Committee))
which will handle it automatically
but need to make sure by default it doesn't fetch the related object - only when user requests it
so I'm not sure what's the best way to implement it..

Begin by locating the dataservice field implementation and any related-object handling; the issue names no files or tests. Clarify the proposed field API and lazy-loading behavior with the maintainers, then define completion as supporting related table fields without fetching the related object by default and fetching it only when requested.

Vom Indexierungsmodell aus dem Issue-Text verfasst.