Publicize: bring in replies and likes back to WordPress

### Impacted plugin

Jetpack, Social

### What

When you use Publicize, your newly published posts are automatically published to your social media accounts, where folks can reply to the post and like that post. It would be nice if their reactions could be brought back to your site and stored as custom comments. It would keep a copy of the conversations on your own site that you control.

### How

Since a lot of the discourse about a blog post happens on social networks nowadays, some of those conversations end up being lost when that social network eventually disappears. It also siloes the conversations in a walled garden.

If possible and if the social network allows it, it would be nice to save those conversations on the WordPress site, as comments with a custom comment type. This would allow site owners to display those conversations and interactions if they want to, and however they would want to.

Of note, this is something that already happens with the ActivityPub plugin for WordPress, where it is named "Federated comments":
https://github.com/Automattic/wordpress-activitypub/

Start by reviewing the Publicize functionality in the Jetpack and Social plugins, then compare the ActivityPub plugin's documented “Federated comments” behavior. Done would mean defining and implementing a supported way to bring social replies and likes back to WordPress as custom comments, but the issue does not name files or tests.

Written by the indexing model from the issue text.