Allow filtering by job-listing author on front-end

Requested here: https://wordpress.org/support/topic/how-to-show-jobs-posted-by-a-particular-user/

It's possible in the admin, as per https://github.com/Automattic/WP-Job-Manager/issues/296

Start by reading the linked WordPress support request and Automattic/WP-Job-Manager issue #296 to understand the existing admin behavior and the requested front-end equivalent. Then locate the front-end job-listing filtering entry point; done means users can filter listings by their author in the front end as described by the request.

Written by the indexing model from the issue text.