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

[github-actions]

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[github-actions]

Closing due to inactivity. If this is still relevant, please reopen or file a new issue.

[jacalata]

Landed as ResourceReference. See reference_item.py; items now compose or inherit from it.