v4 API: Add Subscriber to Legacy Forms / Landing Pages

[n7studios]

Summary

Adds methods to add a subscriber to a legacy Form / Landing Page.

These are undocumented v4 API methods, but we need them for existing Plugin installations that are configured to add subscribers to Legacy Forms / Landing Pages (such as our WooCommerce integration).

Testing

• Test coverage added for testing new methods

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)