[OAS 3.1.0] Callback.expression can now be a pathItem reference as well as an object

[Azquelt]

A Callback expression field can now be a reference to a Path Item, as well as a Path Item object directly.

We likely don't need to do anything here since our PathItem model object already extends Reference (though it probably shouldn't have) but we may want to:

• check if any documentation updates are needed
• add a test which uses a reference here

Note that #437 allows PathItems to be added as components.