Add option to set MailChimp's GDPR fields from Hustle

Hustle currently looks up the interest groups on a MailChimp list and allows you to add people to that group, I would therefore suggest that Hustle also had a way to set the marketing preferences for Mailchimp's GDPR options and allowed you to set the email consent.

The available MailChimp’s GDPR fields are listed in this article (I know that the article refers to MailChimp forms, but you can also set them via the API): https://mailchimp.com/help/collect-consent-with-gdpr-forms/#gdpr+fields

You can customise all those fields but in this case we only want to set the email field.