Contact Form: subscription integrations

Following the addition of the consent field to the form block:


It may make sense to offer more options around the integration with this field. Here is what comes to mind:

```[tasklist]
- [ ] Tie in with [the Subscriptions feature](https://jetpack.com/support/subscriptions/)
- [ ] Publish a post on developer.jetpack.com explaining how third-parties can tie in their own solution with the new meta data available in form submissions.
```

***

Internal references:

> Where are my Newsletter subscribers listed?
> How do I send messages to my Newsletter subscribers?
-- p93VbO-gZ-p2/#comment-679

> The Newsletter form is super handy, but it’s a little confusing, on what it’s doing when someone completes it. At first, I thought somehow we were integrating with the Users of the site, or maybe Jetpack had introduced a Subscriber list of some sort that I missed
-- p8oabR-xs-p2/#comment-4530

***

External feedback:

> If this sounds a bit convoluted, it’s because it is. Without reading the docs or opening the Newsletter Integration tab in the block options, some users may be wondering how this feature works.
-- https://wptavern.com/jetpack-8-9-adds-donations-block-newsletter-form-and-social-previews

Start by reviewing the Contact Form consent-field integration and the linked Subscriptions feature documentation. The issue proposes subscription support and a developer.jetpack.com post explaining third-party use of form-submission metadata; completion would require defining the integration and publishing that guidance. No files or tests are named.

Written by the indexing model from the issue text.