Integrate the Authorship plugin with the Workflows module

Use Case:
As a content creator, I want to be able to send notifications to multiple authors and guest authors who are contributors to the post.

Requirement:
The multiple and guest authorship to work seamlessly in my existing workflows and notification streams.

**Acceptance Criteria:**
- [ ] If the Authorship feature is enabled, all assigned authors should be able to be notified (Basically, it should integrate well with workflow notifications)

(So if the guest author is just a tag, this might not be possible unless the guest author has been added as a user)