Support for Google Tag Gateway

[gusfune]

Goals

Google Tag Manager and Google Analytics have this feature called Google Tag Gateway for Advertisers which would allow a reverse proxy setup for using first-party infrastructure to send data to those services.

Currently @next/third-parties/google doesn't support this feature because a gtmId is required as a parameter for making it work. After testing it's confirmed that this field is breaking the GTM usage when running the first-party proxy due to the url param appended to the url ?id=

Non-Goals

Won't implement a full reverse proxy, this is an easy feature that developers can do themselves as they see fit, as it doesn't need to be done necessarily from inside NextJS, but also at network level

Background

As documented in: https://developers.google.com/tag-platform/tag-manager/gateway/setup-guide?setup=manual

Proposal

A simple patch to change the parameters to make gtmId as an optional prop and gtmScriptUrl required if gtmId is missing.

[gusfune]

I volunteer to make the change, just reporting this here as this is a requirement on contribution docs.

This is already made into a PR: #81011