Mailchimp block: Support extra fields like first name and last name

We had a report that it would be a good idea to have extra fields on the block, for visitors to enter their first name and last name. Is that something that Mailchimp might support, and something we could implement?

#11140992-hc (#1867222-zen)

#user-report

Start by locating the Mailchimp block implementation and checking how its current visitor fields are sent to Mailchimp. Confirm whether first_name and last_name are supported, then define the field behavior and verify the completed block submission before considering the issue done.

Written by the indexing model from the issue text.