Add "Google consent mode" handling

It would be great if this module could accommodate the Google Consent mode (beta) directly as per the CivicUK blog post.

Following the example from that post, it should be as straightforward as:
1. Adding a checkbox in the admin dashboard to enable/disable "Google Consent mode".
2. Include documentation about the accompanying accept / revoke code that should be in the purpose categories.
3. If "Google Consent mode" is enabled, adding the following script as the very first script on the page:

  window.dataLayer = window.dataLayer || [];
    function gtag() {
      dataLayer.push(arguments);
    }

    // Default values for consent mode is 'denied'.
    gtag("consent", "default", {
      ad_storage: "denied",
      analytics_storage: "denied"
    });

A. Do the maintainers think this belongs in this module?
B. Does the approach above sound correct?