YouTube Extractor: On collaborative playlists add support for "Added by" field in --write-info-json

[akaleeroy]

Checklist

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2019.06.08
• I've searched the bugtracker for similar site feature requests including closed ones

Description

Collaborative YouTube playlists have a #contributor element for every <ytd-playlist-video-renderer> that says Added by Someone.

This is a request to also extract this information when producing the output of --write-info-json.
Perhaps something like:

"contributor_name": "Someone"
"contributor_url": "https://www.youtube.com/channel/someone"

I haven't yet been able to understand how the extraction happens, if it's simple to add this or not.

Thank you so much for this project!