FEATURE: Experimental support for group membership via google auth (#14835)

This commit introduces a new site setting "google_oauth2_hd_groups". If enabled, group information will be fetched from Google during authentication, and stored in the Discourse database. These 'associated groups' can be connected to a Discourse group via the "Membership" tab of the group preferences UI. 

The majority of the implementation is generic, so we will be able to add support to more authentication methods in the near future.

https://meta.discourse.org/t/managing-group-membership-via-authentication/175950

lib/validators/google_oauth2_hd_groups_validator.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class GoogleOauth2HdGroupsValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(value)
+    @valid = value == "f" || SiteSetting.google_oauth2_hd.present?
+  end
+
+  def error_message
+    I18n.t("site_settings.errors.google_oauth2_hd_groups") if !@valid
+  end
+end