dataservice should support unified field for id / desc fields from new knesset API

@OriHoch:
> the new knesset API have a lot of these fields where you have somethingID and related somethingDesc
worth to create a new KnessetDataServiceField class which will handle it automatically -
so instead of defining those two fields, you would define only 1 field:
("type", KnessetDataServiceIdDescriptionField("SubType", "type of the bill"))
behind the scenes this field can expose a python tuple of (id, description)

Start by locating the existing KnessetDataService field definitions and how paired somethingID and somethingDesc values are handled. Define the scope of KnessetDataServiceIdDescriptionField and verify that a single field can expose the API pair as an (id, description) tuple; done means the proposed SubType definition works without separate fields.

索引モデルが issue の本文から書いたものです。