Add integration with WP Consent API

[martynmjones]

Changes proposed in this Pull Request:

Closes #424

Adds integration with the WP Consent API plugin to better support different consent banner configurations.

If the plugin is installed on a website then the existing consent state will be sent as an update when tracking is initialised. For on-page updates, we're adding an event listener for wp_listen_for_consent_change which is dispatched by WP Consent API when any changes are made.

Detailed test instructions:

General test instruction

1. Build extension from add/424-integration-with-wp-consent-api
2. Install Complianz and setup a basic consent banner
3. Debug the site using Tag Assistant while logged out
4. Grant consent via consent banner
5. Confirm a Consent event is sent with the updated state
6. Trigger events on page i.e visit the shop archive, visit a single product, add a product to cart
7. Confirm the events are all recorded in Tag Assistant and that the consent On-page Update is Granted (Under the Consent tab)
   
8. Revoke consent via the consent banner
9. Confirm that the consent state is no longer Granted in Tag Assistant

Event order testing

By default, the WP Consent API plugin JavaScript loads in the footer meaning we are unable to send the consent update event with the current state until after the page loads.

We need to support calls to gtag from on the page for Google Listings & Ads and other third-party extensions. To do that we're including the wait_for_update parameter as recommended by Google so that events sent before the consent update will still be recorded. 57ec41e

1. Trigger an event on the page

add_action(
	'wp_head',
	function() {
		echo '<script>gtag( "event", "example_event" );</script>';
	}
);

2. Visit the site while debugging via Tag Assistant with consent Granted
   1. Confirm the event is sent before the consent update
   2. View the Consent tab and confirm that the On-page Update is listed as Granted

Without wait_for_update:

With wait_for_update:

Additional details:

• Another option was to add the existing consent state to the ga4w data object so that immediately after we set the consent defaults the update can be sent. While this would be preferable, caching and minification plugins would cause problems with it which is why we're relying on a purely JS implementation instead
• Tests will be added in a follow-up PR

Changelog entry

Add - Integration with the WP Consent API plugin

[tomalec]

I run tests locally - works well. However, initially, I get some quirk that WP Consent API's cookie was not created after allowing in Complianz modal. Restarting new incognito mode helped. I'll take some time to try to reproduce it, not to miss something.

I left some comments regarding the code. The most important are marked with 🚧

[martynmjones]

Hey @tomalec, thanks for the review 🙌

I've made some adjustments and responded to your comments so this is ready for a second round.

[tomalec]

After adding the fixes I suggested, I tested the branch with the following extensions:

1. Complianz, w/ & w/o Autooptimize
2. Cookie bot, w/ & w/o Autooptimize
3. GDPR Cookie Compliance, w/ & w/o Autooptimize

I added the WP Consent API steps to general testing wiki https://github.com/woocommerce/woocommerce-google-analytics-integration/wiki/General-Testing#12-wp-consent-api-integration

I also created Setup & Smoke testing instructions for those three extensions in a separate Wiki: https://github.com/woocommerce/woocommerce-google-analytics-integration/wiki/Smoke-Testing-WP-Consent-API-integration-with-other-extensions. I know we don't have the capacity to maintain support for all of them. Still, IMHO, it would be good to at least one integration works.

I made a PR with a small README note about WP Consent API and UI for banners.
#426

---

Tests will be added in a follow-up PR

Do you have anything started, or can I pick that up?

[martynmjones]

Hey @tomalec, many thanks for the additional testing and for updating the documentation. I've made some changes based on our discussions so far if you could please take another look.

[tomalec]

Reviewed the code, tested locally. Using Polish IP, and US one, tested with two cookie banner plugins and fiddling manually with API.

LGTM