Provide Webform handler

Problem/Motivation

A website can have multiple other forms where you could add a Subscribe to Newsletter feature. At the moment this module only provides a simple Subscribe form.

It would be very useful to provide a Webform handler so you can create subscribers from regular Webform submissions. This also allows a website owner to retroactively add or replace a newsletter subscription to existing webforms.

Remaining tasks

Create a Webform handler with basic field & list mapping and expand the API call to support the extra fields.

User interface changes

Managed as a regular Webform handler, no extra UI changes needed.

API changes

Possibly expand API call to support the extra fields.

Data model changes

None, managed via the Webform handler functionality.