Introduce a (XWiki)Document#setAuthor(AuthorReference) helper covering most use case of document author set

The idea is to provide an API to cover the most common use case by far when you want need to update the author of the document depending on what changed in the document without to care yourself too much on the various types of authors.

See https://forum.xwiki.org/t/introduce-an-author-setter-helper-in-xwikidocument-for-the-most-common-use-case/13628 for more details.