Add gtag consent mode v0 support

[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)

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

Screenshots:

Detailed test instructions:

1. Build, install, activate the extension
2. Deactivate Google Analytics Integration extension
3. Open Tag assistant
4. Visit shop page
5. You should see consent mode set up
   
6. Read the docs, check if they are explanatory enough
7. Try the following snippet

add_filter(
    'woocommerce_gla_gtag_consent',
    function( $old_config ) {
        $additional_config = "
        gtag( 'consent', 'default', {
            analytics_storage: 'granted',
            ad_storage: 'granted',
            ad_user_data: 'denied',
            ad_personalization: 'denied',
            region: ['PL'], // <--------- SET IT TO YOUR REGION
        } );
        ";
        return $old_config . $additional_config;
    }
);

9. You should see another consent entry specified
   

Additional details:

Changelog entry

Add - Google Analytics consent mode support.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 63.8%. Comparing base (f9a5637) to head (de77d2b).

Additional details and impacted files

@@                                Coverage Diff                                 @@
##             update/global-site-tag-for-analytics-version-2   #2290     +/-   ##
==================================================================================
- Coverage                                              63.9%   63.8%   -0.0%     
- Complexity                                             4196    4197      +1     
==================================================================================
  Files                                                   455     455             
  Lines                                                 17853   17859      +6     
==================================================================================
  Hits                                                  11400   11400             
- Misses                                                 6453    6459      +6     

Flag	Coverage Δ
php-unit-tests	63.8% <0.0%> (-<0.1%)	⬇️

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

[martynmjones]

Hey @tomalec, thanks for your work on consent in GLA!

I've left two comments on the docs but all is working well so LGTM ✅