MailchimpSignupSubscribeBlock prevents Dynamic Page Cache due to direct form embedding

Problem/Motivation

The MailchimpSignupSubscribeBlock directly embeds a form using $this->formBuilder->getForm() in its build() method. Forms contain unique security tokens (form_build_id and form_token) that must be generated fresh on every request, which prevents Dynamic Page Cache from caching any page containing this block.

This impacts site performance, as pages with the subscribe block cannot be served from Dynamic Page Cache, requiring full rendering on every page load.

Steps to reproduce

1. Place a Mailchimp Signup block on a page
2. Enable Dynamic Page Cache module
3. Check response headers - you'll see X-Drupal-Dynamic-Cache UNCACHEABLE (poor cacheability) on every request
4. The page is never cached due to the form tokens

Proposed resolution

Convert the form rendering to use a lazy builder, which allows the block content to be cached while the form (with its unique tokens) is rendered as a placeholder after cache retrieval.