Disable "anonymous" commenting via Jetpack Comments

#### Is your feature request related to a problem? Please describe.
I would like to require a minimal level of accountability for users leaving comments on my sites.

#### Describe the solution you'd like
I would like the ability to require users that want to leave a comment to log in, either to my site, or to one of the supported social networks (presently Google, Facebook, Twitter, wordpress.com).
[https://bureau42.com/wp-content/uploads/2021/02/Screenshot-2021-02-22-151935.png](https://bureau42.com/wp-content/uploads/2021/02/Screenshot-2021-02-22-151935.png)

#### Describe alternatives you've considered
* WordPress has a native setting to require users to log into the local site before commenting, but that imposes a site-specific barrier to leaving comments.
* Allowing anonymous users to comment leads to comment spam, which requires other plugins (like Akismet) and introduces additional complexity to managing a site.

The issue names no files or tests. Start by reviewing the Jetpack Comments component and its existing Google, Facebook, Twitter, and WordPress.com login options. Done means site owners can disable anonymous comments while retaining authenticated commenting through the supported options.

Written by the indexing model from the issue text.