Add support to GTM and GA4 for First-Party mode

[dja]

What does this PR do? This allows for using Google's First-party mode, to load Google scripts from your own server through server-side tagging.

Are there breaking changes in this PR? No

Testing

Any background context you want to provide? This capability helps ensure that Google Tag Manager domain and script loading is not impeded by browsers, and Segment users can setup the First-party mode to support this.

Is there parity with the server-side/android/iOS integration components (if applicable)? N/A

Does this require a new integration setting? If so, please explain how the new setting works

• Yes, in the Google Analytics 4 Web and Google Tag Manager destinations, the new setting is domain, which defaults to www.googletagmanager.com

Links to helpful docs and other external resources

https://developers.google.com/tag-platform/tag-manager/server-side/dependency-serving?tag=gtag