Implement consent mode v0

[tomalec]

Changes proposed in this Pull Request:

Add the minimalistic implementation to add support for Google Consent mode

We want to add MVP ASAP before Google starts penalizing merchants without this setup. (pcTzPl-25o-p2)
Later, we could iterate by adding settings to customize the snippet a bit using UI.

This PR adds a basic default and documents how to customize/extend it.

Checks:

• Does your code follow the WordPress coding standards?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Screenshots:

Detailed test instructions:

1. Buiild, Install and activate the extension
2. Deactivate GLA (if applicable, as the integration is broken for remove-universal-analytics branch)
3. Disable "Add to cart" events in settings (/wp-admin/admin.php?page=wc-settings&tab=integration&section=google_analytics) so WooCommerce Core will not set up its own gtag config making us do nothing.
4. Open Tag Assistant
5. Open shop page the page
6. In Tag assistant you should see the "Consent" as the first entry.
7. If you're connecting from EEA region, you should see "Consent" being Denied by default
   
8. If you're connecting from non-EEA region, you should see "Consent" not set
   
9. Check that Readme description
10. Add the snippet to customize the consent mode:

add_action( 'wp_enqueue_scripts', function () {
    $code = "
        gtag( 'consent', 'default', {
            analytics_storage: 'granted',
            ad_storage: 'granted',
            ad_user_data: 'denied',
            ad_personalization: 'granted',
            region: 'PL', // <--------- SET IT TO YOUR REGION
        } );";

    wp_register_script( 'my-custom-consent-mode', '', array('woocommerce-google-analytics-integration'), null, [ 'in_footer' => true ] );
    wp_add_inline_script( 'my-custom-consent-mode', $code );
    wp_enqueue_script( 'my-custom-consent-mode' );
} );

Check that the second Consent entry is added with custom defaults

Additional details:

1. This customization capabilities are not ideal, as the is a change to get gtag hit before the second consent config is saved, as we may trigger page_view or other events. We may improve that in the future.

   Especially since the current logic to force main.js to run after the document is ready is pretty hacky. (I guess that's why we add inline scriptonwp_footer` hook). I think we can make it in a cleaner fashion:

   • wrap all JS code that requires document ready in DOMContentLoaded event listener
   • execute the code that needs to run synchronously

Changelog entry

Add - Basic consent mode support.

[tomalec]

After #363 the [ 'in_footer' => true ] part would not be needed in the snippet.

[martynmjones]

Hey @tomalec, thanks for adding the initial consent implementation.

All appears to be working well and the consent parameters are picked up by Google. One issue with the country code config but everything else looks good ✅