Mailchimp block: add option to pick your own audience group from the front-end

The Mailchimp block includes options for the post author to pick an audience group for all visitors signing up using that block.

However, the Mailchimp docs suggest a different approach:
> Groups are specialized audience fields that let contacts **self-categorize** based on interests or preferences using uniform responses you set.
-- https://mailchimp.com/help/create-new-audience-group/

The keyword there is "self-categorize", that is, the person who's signing up for the email list should themselves be able to select the group they want to be in (if the list has groups).

We could adapt our block to surface the group options in the actual block to let the visitors choose.

Start at the Mailchimp block's front-end implementation and trace how its current audience-group option is exposed to visitors. Define the visitor selection behavior from the issue and verify that a signup can use the selected group, with tests covering the available groups and submitted choice.

Written by the indexing model from the issue text.