Add JSON key and option for unlisted/private videos

[mrnoname1000]

Checklist

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2020.06.06
• I've searched the bugtracker for similar site feature requests including closed ones

Description

When downloading videos from a YouTube playlist, there's no way to pick out the unlisted ones. Looking at youtube-dl's JSON output, there seems to be no indicator that a video is unlisted. There seems to be no indicator for private videos either, although I haven't tested this.
Adding a "visibility" entry or something similar to the JSON output would solve this. I imagine the possible values would be "visibility": "public", "visibility": "unlisted", and "visibility": "private".
Adding an option to filter downloads by visibility would also be nice, i.e. --visibility private or --visibility public,unlisted.