Add support to CoAuthor Plus' guest authors

[leogermani]

Currently, Yoast integration with Co Authors Plus supports displaying multiple authors for a post, but it does not support Guest Authors.

This PR, which is still work in progress, adds support to Guest Authors.

• Test instructions *

• Enable Yoast and CoAuthors Plus

• Create a post

Now try different authorship states and check the resulting schema:

• only one regular author
• 2 or more authors who are regular users
• mix of regular users and guest authors as authors of the post
• only one guest author as the post author
• multiple guest authors as post authors

Also test guest authors with and without avatars, and with and without emails with gravatars. Check that the image tag is properly added in all cases.

• Known issues *

This seems to be working fine for singular pages, but not working properly for author archives

What other use cases are missing?