Support workflow method to request for which branch the current draft/unpublished/published variant is

See step (3) from REPO-2032 which says:

"3) Clicking 'Add to project' in the Channel Mgr in the context of Branch X in case the current unpublished variant is for Branch Y, should first checkout the Master, and then create the branch
For step (3) we most likely also need a DocumentWorkflow that tells which branch the current unpublished branch is for, since we do not yet have that. I'll create a separate issue for this"

Most likely, in the Channel Mng we need to be able to request to which branch the current unpublished variant belongs