Use service account credentials for fetching google hd groups (#18329)

The previous implementation would attempt to fetch groups using the end-user's Google auth token. This only worked for admin accounts, or users with 'delegated' access to the `admin.directory.group.readonly` API.

This commit changes the approach to use a single 'service account' for fetching the groups. This removes the need to add permissions to all regular user accounts. I'll be updating the [meta docs](https://meta.discourse.org/t/226850) with instructions on setting up the service account.

This is technically a breaking change in behavior, but the existing implementation was marked experimental, and is currently unusable in production google workspace environments.

lib/validators/google_oauth2_hd_groups_validator.rb

@@ -6,7 +6,7 @@ class GoogleOauth2HdGroupsValidator
   end
 
   def valid_value?(value)
-    @valid = value == "f" || SiteSetting.google_oauth2_hd.present?
+    @valid = value == "f" || (SiteSetting.google_oauth2_hd.present? && SiteSetting.google_oauth2_hd_groups_service_account_admin_email.present? && SiteSetting.google_oauth2_hd_groups_service_account_json.present?)
   end
 
   def error_message