Add add_subscriber_to_form() function with named arguments

[n7studios]

Summary

Adds the add_subscriber_to_form() API function, to include support for first name, custom fields and tags, per https://developers.convertkit.com/#add-subscriber-to-a-form.

Outputs a PHP deprecated notice if using the existing form_subscribe(), function, as its second argument supports an array of options, whilst add_subscriber_to_form() provides the specific named arguments (form ID, email address, first name, custom fields), as we do for e.g. add_subscriber_to_sequence().

Adds tests to improve test coverage of new function arguments.

Testing

• testAddSubscriberToForm: Test that adding a subscriber to a form works
• testAddSubscriberToFormWithInvalidFormID: Test that adding a subscriber to an invalid form ID throws a ClientException
• testAddSubscriberToFormWithInvalidEmailAddress: Test that adding a subscriber to a form with an invalid email address throws a ClientException
• testAddSubscriberToFormWithFirstName: Test that adding a subscriber to a form with a first name specified works.
• testAddSubscriberToFormWithCustomFields: Test that adding a subscriber to a form with a custom field works.
• testAddSubscriberToFormWithTagID: Test that adding a subscriber to a form with a tag ID works.

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)