Review Board 4.0 is introducing metadata on expanded fields, in order to

help determine which fields have been expanded and what their original

resource mimetypes/URLs would be.
This change adds this support to RBTools. Now, when accessing an

expanded field (and running with Review Board 4.0), the caller will see

a resource object rather than a simple field wrapper. As field access

behaves the same on both, this should be backwards-compatible, and will

now allow callers to invoke any methods that would normally be on the

resource.
Currently, we don't return custom list resource subclasses, as the

current ListResource expects to have a full payload containing item

counts and a nested list of items. We can revisit this down the road

if we begin to need this functionality, and are already set up to do so

in the field wrapping mechanisms.

Made use of this support in the new rbt patch code that's in development.
Unit tests pass.