Comments: Add support for comment_form_top action after comment title

#### Is your feature request related to a problem? Please describe.
WordPress Core supports the `comment_form_top` action to allow content to be added at the top of the comment form.

Developer Docs: https://developer.wordpress.org/reference/hooks/comment_form_top/
Source: https://github.com/WordPress/WordPress/blob/5.6/wp-includes/comment-template.php#L2555-L2561

```
function comment_add_policy() {
echo esc_html( '

Howdy, by leaving a comment you agree to our comment policy.

' );
}
add_action('comment_form_top', 'comment_add_policy');
```
As Jetpack replaces the comment form and the action is not available, but does appear at the bottom of the comment form but not at the reply location.
https://github.com/Automattic/jetpack/blob/9.2.1/modules/comments/comments.php#L327

#### Describe the solution you'd like
Either adding support for `comment_form_top` within the Jetpack comment form by adding `do_action( 'comment_form_top' )` after the greeting `if` statement but before the iframe:
https://github.com/Automattic/jetpack/blob/9.2.1/modules/comments/comments.php#L338

Alternatively, a new filter could be added `jetpack_comment_form_top` which would allow for customization but be specific to Jetpack itself.

#### Describe alternatives you've considered
Using JavaScript to modify the DOM to add the message before the iframe.

The requested integration is in modules/comments/comments.php around the greeting if statement and iframe, with the issue linking the relevant lines. First inspect that comment-form path and the existing action behavior. Done means the WordPress comment_form_top hook is available at the requested position without relying on JavaScript, or that the selected Jetpack-specific alternative is clearly implemented.

Written by the indexing model from the issue text.