v4 API: Support Adding Subscribers to Legacy Forms

[n7studios]

Summary

Uses this PR and some logic to call the applicable v4 API method, depending on whether the Plugin needs to add the subscriber to a Form or Legacy Form.

Testing

• testSettingsContactForm7ToConvertKitLegacyFormMapping: Test that Contact Form 7 to Legacy Form subscribing/mapping works
• testSettingsForminatorFormToConvertKitLegacyFormMapping: Test that Forminator to Legacy Form subscribing/mapping works
• testSettingsWishListMemberLevelToConvertKitLegacyFormMapping: Test that WishList Member to Legacy Form subscribing/mapping 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)

[noelherrick]

I'm fine with a bit of duplication, but perhaps we add a new method in the core library that subscribes to either a non-legacy or a legacy form? I think that could also reduce the number of tests you need to write since we can just test it in the library and then the consumers can just call it transparently.

[n7studios]

I'm fine with a bit of duplication, but perhaps we add a new method in the core library that subscribes to either a non-legacy or a legacy form? I think that could also reduce the number of tests you need to write since we can just test it in the library and then the consumers can just call it transparently.

I considered this, however this would involve an additional API call. Forms are cached by each Plugin via a resource class, and the core library won't know which resource class to reference (e.g. ConvertKit_Resource_Forms, CKWC_Resource_Forms etc). The core library would need to look up whether the Form ID given is legacy or not before determining which API endpoint to call.

That aside, the concept of "subscribing" to a form / legacy form is from the v3 API, as there was no way to create a subscriber (unless subscribing to a tag or other resource). Now that the v4 API provides the ability to create a subscriber, I'd like to remove this concept of subscribing to a form in the Plugin (just not at the same time as migrating to the v4 API).

All calls made to form_subscribe (and legacy_form_subscribe) relate to our integrations with third party Form Plugins. Subscribing customers to a form in ConvertKit when they're filling out a form in WordPress confuses some customers, and is a concept we can move away from in the future, given the ability to directly create a subscriber.

As such, this code exists to ensure we don't introduce breaking changes, but will later be deprecated and then removed.