Add add_subscribers_to_forms method

[n7studios]

Summary

Adds the add_subscribers_to_forms method, to support bulk subscribing subscribers to forms:
https://developers.kit.com/v4.html#bulk-add-subscribers-to-forms

Testing

• testAddSubscribersToForms: Test that add_subscribers_to_forms() returns the expected data.
• testAddSubscribersToFormsWithReferrer: Test that add_subscribers_to_forms() returns the expected data when a referrer URL is specified.
• testAddSubscribersToFormsWithReferrerUTMParams: Test that add_subscribers_to_forms() returns the expected data when a referrer URL with UTM parameters is specified.
• testAddSubscribersToFormsWithInvalidFormIDs: Test that add_subscribers_to_forms() returns the expected errors when invalid Form IDs are specified.
• testAddSubscribersToFormsWithInvalidSubscriberIDs: Test that add_subscribers_to_forms() returns the expected errors when invalid Subscriber IDs are specified.

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
• The code passes when running PHPStan
• 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)