feat(gatsby-plugin-google-tagmanager): add option for selfHostedOrigin

[tackaberry]

Description

In the gatsby-plugin-google-tagmanager, this change supports variable selfHostedOrigin, defaulting to googletagmanager.com.

With the advent of Server Side Tagging, we have the ability to serve gtm.js from "self-hosted" tagging server.
See: https://developers.google.com/tag-manager/serverside
And: https://developers.google.com/tag-manager/serverside/send-data#update_the_gtmjs_source_domain

Thus the base url, instead of googletagmanager.com could be sst.example.com or analytics.example.com.

Follow up to discussion: #32719

With selfHostedOrigin: 'https://sst.example.com':

Default:

Documentation

There is a related change to the README in the package.

[tackaberry]

@wardpeet Can you help determine why the tests aren't passing? I can't decipher any reasons why this is being stopped.

[wardpeet]

Can you add tests as well? You also have to make it a little bit more robust so origin never ends with a /.

You can skip pnp test for now.

[tackaberry]

@wardpeet thanks for the notes. Tests added, and change changed.

[tackaberry]

hey there @wardpeet - Im curious about the process from here- I'd like to get this feature in use as a plugin if we can. Anything else I can do to push this forward?

[wardpeet]

Hey @tackaberry, sorry this was forgotten on my end. I've updated the branch with latest master and will go over it one last time and merge.

[tackaberry]

Great. Thanks for merging in master. Anything I have to do at this point? How long typically until it gets merged in and available in the release?

[wardpeet]

@tackaberry normally only a day when things are good to go. We publish it under the @next tag. So you can play with it with npm install gatsby-plugin-google-tagmanager@next. I'll comment when it's released