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

gatsbyjs#32733)

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: developers.google.com/tag-manager/serverside
And: developers.google.com/tag-manager/serverside/send-data#update_the_gtmjs_source_domain

Co-authored-by: Ward Peeters <ward@coding-tech.com>

yarn.lock

@@ -1869,16 +1869,21 @@
     "@types/node" "*"
     form-data "^3.0.0"
 
-"@types/node@*", "@types/node@^8.5.7":
-  version "8.10.66"
-  resolved "https://registry.yarnpkg.com/@types/node/-/node-8.10.66.tgz#dd035d409df322acc83dff62a602f12a5783bbb3"
-  integrity sha512-tktOkFUA4kXx2hhhrB8bIFb5TbwzS4uOhKEmwiD+NoiL0qtP2OQ9mFldbgD4dV1djrlBYP6eBuQZiWjuHUpqFw==
+"@types/node@*":
+  version "16.4.12"
+  resolved "https://registry.yarnpkg.com/@types/node/-/node-16.4.12.tgz#961e3091f263e6345d2d84afab4e047a60b4b11b"
+  integrity sha512-zxrTNFl9Z8boMJXs6ieqZP0wAhvkdzmHSxTlJabM16cf5G9xBc1uPRH5Bbv2omEDDiM8MzTfqTJXBf0Ba4xFWA==
 
 "@types/node@^14.14.10":
   version "14.17.12"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-14.17.12.tgz#7a31f720b85a617e54e42d24c4ace136601656c7"
   integrity sha512-vhUqgjJR1qxwTWV5Ps5txuy2XMdf7Fw+OrdChRboy8BmWUPkckOhphaohzFG6b8DW7CrxaBMdrdJ47SYFq1okw==
 
+"@types/node@^8.5.7":
+  version "8.10.66"
+  resolved "https://registry.yarnpkg.com/@types/node/-/node-8.10.66.tgz#dd035d409df322acc83dff62a602f12a5783bbb3"
+  integrity sha512-tktOkFUA4kXx2hhhrB8bIFb5TbwzS4uOhKEmwiD+NoiL0qtP2OQ9mFldbgD4dV1djrlBYP6eBuQZiWjuHUpqFw==
+
 "@types/parse-json@^4.0.0":
   version "4.0.0"
   resolved "https://registry.yarnpkg.com/@types/parse-json/-/parse-json-4.0.0.tgz#2f8bb441434d163b35fb8ffdccd7138927ffb8c0"