Include observation creator information via the API

The web view displays the creator username of a particular observation with the label "Observer", which is stored in the field author. While stored as author it is commonly also called observer.

This information is not available at the /api/observations API endpoint.

Related user information is available as waterfall_status_user and the deprecated vetted_user fields returned by the API already.

Including who created the observation is useful for e.g., giving credit to someone for scheduling an "interesting" observation, or extracting via the API various statistics about the implied scheduling priorities of users. The information is already public via the web view, but an analyst wanting to make use of that information must currently scrape the HTML.