Authorization Policy vNext -> Nested Table References

To further enhance the authorization policy capability, add the ability to map to nested tables/columns. For example:

- Database Policy: Entity (book) `"New Starch Press" eq @item().publisher_id.name`

Because the policy is applied to the book entity, the nested policy will search the Publishers table using publisher_id to acquire the record where name column is "New Starch Press"

Nesting will apply to the `@item` "Policy Expression System Object" described in the [request authorization RFC](https://github.com/Azure/project-hawaii/blob/main/rfcs/request-authorization.md#policy-expression-system-objects).