Allow client-side storing of comment meta

[iandunn]

As a modern counterpart to #12431, I think it'd be nice if plugins could add comment meta from the client side as well.

An example use case would be ​a plugin that identifies toxic comments, and then encourages the author to rephrase it to be more kind before it gets submitted.

It's currently possible by adding a preprocess_comment callback that examines $_POST, verifies and sanitizes the data, and then adds it to $commentdata before that gets passed to wp_insert_comment(). It could be much easier and standardized, though.

Ideally, I think a client-side script should be able to just append a hidden input field named comment_meta[] to the .form-submit element, and have it be automatically added via wp_insert_comment(), rather than being ignored by wp_handle_comment_submission().

Since it's coming from the client side, the data can't be trusted. We need to verify that it was added by the enqueued script, and then sanitize it. Maybe there could be something similar to register_post_meta() with a allow_in_form boolean param, and a sanitize_callback param.