Webform newsletter subscription integration checkbox & handler

Postponed on ✨ Brevo newsletter (un)subscribe API Active

Problem/Motivation

Webforms are super popular in Drupal and Brevo module should provide webform integration in a brevo_webform submodule.

It should allow adding a checkbox element to subscribe to the selected newsletter on form submission. Maybe this also needs a dedicated webform handler, I'm not sure about the most flexible architecture here. Maybe this is already solved for other newsletter modules?

It might make sense to sponsor @jrockowitz (see https://www.drupal.org/project/webform → ) as Brevo for a smart implementation (and because he and open source is worth the donation!)

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes