Add "as_reference" to all items

[t8y8]

We introduced the concept of a ResourceReference we should add it to all types (via inheritance or not is TBD. Current implementation is not.)

\cc @RussTheAerialist @gaoang2148 @shinchris

[jacalata]

This is currently implemented only for group/groupset/user types - I think that's just because that's where someone needed it first. It would be good to combine this with the Resource class in tableau_types.py