[Open UI] For document field extensions, provide external link to document

Currently when developing an Open UI document field extension, a developer has the option of getting the urlName property for the current document through the ui.document.get() method in the client library. However, the urlName property gives the relative url for the document itself, e.g. 'my-document' for a document called My document (this is fairly useless because the parameter doesn't include the full path of the document in the content tree, but that is somewhat beside the point).

It would be useful if a developer could also retrieve the CMS url for the document (e.g. www.example.com/cms/content/path/content/documents/somechannel/banners/banner1). The use case is to be able to register a document with a DAM system, to indicate that a certain image is used in that document, and be able to provide a link back from the DAM to the document in the CMS.