Mailchimp Block: Support double opt-in

#### Steps to reproduce

Currently, it's not possible to configure double opt-in with Mailchimp block.

1. Set up a list with double opt-in option in Mailchimp. My link to set it up looks like this https://us19.admin.mailchimp.com/lists/settings/defaults?id=xxxx
2. Set up this list with a Mailchimp block
3. Add an email to this list.
4. This email was added to the list without any double opt-in email.

#### What I expected

A double opt-in email.

#### What happened instead

This email was added to the list without any double opt-in email.

#### Context / Source

Tix: 2704143-zen

Start with the Mailchimp block and reproduce the listed setup using a Mailchimp list configured for double opt-in. Trace how the block adds an email to the list, then verify that the completed behavior sends a double opt-in email rather than adding the subscriber immediately.

Written by the indexing model from the issue text.