Add integration with WP Consent API

[martynmjones]

Changes proposed in this Pull Request:

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/2387-wp-consent-api-integration
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 GLA tracking events on page
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

Additional details:

Copied and modified slightly from woocommerce/woocommerce-google-analytics-integration#425

Changelog entry

Add - Integration with the WP Consent API plugin

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 64.3%. Comparing base (340cb89) to head (210333d).
Report is 17 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##             develop   #2425      +/-   ##
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+ Coverage       63.4%   64.3%    +0.9%     
- Complexity         0    4281    +4281     
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  Files            321     780     +459     
  Lines           5027   21910   +16883     
  Branches        1218    1218              
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+ Hits            3188   14097   +10909     
- Misses          1672    7646    +5974     
  Partials         167     167              

Flag	Coverage Δ
js-unit-tests	63.4% <ø> (ø)
php-unit-tests	64.6% <0.0%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/Google/GlobalSiteTag.php	0.0% <0.0%> (ø)

... and 458 files with indirect coverage changes

[tomalec]

Thanks for implementing it @martynmjones .

I tested it with Complianz and GDPR Cookie Compliance :)

I'm not sure if you already worked on it, but to save some time, I made a branch with E2E tests moved from GA4W
https://github.com/woocommerce/google-listings-and-ads/compare/add/2387-wp-consent-api-integration...dev/add-consent-tests?expand=1

[tomalec]

also added some docs notes https://github.com/woocommerce/google-listings-and-ads/compare/add/2387-wp-consent-api-integration...tweak/add-consent-api-docs?expand=1

Feel free to merge those PRs or disregard them if you have a better implementation.

[tomalec]

I also made changes to the public-faceing docs as for GA4W.

[martynmjones]

Hey @tomalec, many thank for adding the tests and docs 🙌 I've merged your branch into this one.

I tested it with Complianz and GDPR Cookie Compliance :)

Thanks for testing! I've not made further changes but have published the PR properly and requested a review to confirm were good to merge into develop.

[tomalec]

Reviewed the code, tested locally with Complianz and GDPR Cookie Compliance, LGTM :)