Add PDF url field to object/invoice crud

[jcullison]

Currently the object/invoice retrieve crud call gives the "body" field with the latest PDF in it.  It would be nice to see the URL to download the PDF there as well, and maybe even a view of this same object that does not include the "body", as the body is not always what is wanted, and it's a couple hops through InvoiceFiles and Files to get that url otherwise.

[lukasz]

@jcullison, thanks for this, and you're right, we'll move to this more standard REST model with new API's. However, we're unlikely to change that in the existing API's, so as not to break backward compatibility.

[jcullison]

One pattern to do this would be to add a url param or a path param to choose which sort of body you get, something like what account and account summary already do.  Different levels of detail would be very nice and shouldn't be a breaking change as it would be adding a new url or url param to the API.

That new url would be: https://rest.zuora.com/v1/object/invoice/summary or https://rest.zuora.com/v1/object/invoice?view=summary