Add publishedBy to the published document and show it in info

Per CMS-15254, property hippostdpubwf:publishedBy was introduced and stored only in version history, then shown in the revision history dialog.

We can easily add the property to the published document variant, similar to the publication date and then show it in the document information dialog and anywhere else we like.
E.g.

This code should work in the workflow:
org.onehippo.repository.documentworkflow.task.ConfigVariantTask#doExecute

if (isApplyPublished()) {
  getVariant().setPublicationDate(new Date());
  getVariant().setPublishedBy(getWorkflowContext().getUserIdentity()); // introduce #setPublishedBy too
}