[gatsby-plugin-google-tagmanager] - Add option to load GTM via a proxy URL instead of www.googletagmanager.com/gtm.js

[michaelzhu23]

Add an option to load GTM via a proxy custom URL instead of the default www.googletagmanager.com/gtm.js. This is more for those who have set up a server-side GTM container with a custom subdomain. As server-side tracking is becoming more and more prevalent and necessary, I can see this becoming a widely requested feature.