Contact Form: add option to send emails from WordPress.com

When using Jetpack's Contact form feature, emails are sent to the form author whenever someone submits data in the form on the site. Those emails are sent directly from the site, using WordPress' `wp_mail()`.

While this works in most cases, it can have some issues:

1. Some servers are not configured to be able to send out emails. Folks then have to rely on SMTP plugins to be able to send out emails from their site.
2. Emails sent from a shared hosting environment can sometimes have deliverability issues, so the emails never make it to the form author's inbox.

One way around those problems would be to send the emails from the WordPress.com platform instead, like we do for Subscription emails today.

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Related issue: #32027.

Start by tracing Jetpack’s Contact form email path from the use of WordPress’s wp_mail(), and review related issue #32027. The issue provides no file or test location; done would mean defining and implementing an option for submissions to send emails through WordPress.com while retaining the existing behavior.

Written by the indexing model from the issue text.