FEATURE: Custom unsubscribe options (#17090)

With this change, plugins can create custom unsubscribe keys, extend the unsubscribe view with custom preferences, and decide how they are updated.

lib/discourse_plugin_registry.rb

@@ -95,6 +95,8 @@ class DiscoursePluginRegistry
 
   define_filtered_register :notification_consolidation_plans
 
+  define_filtered_register :email_unsubscribers
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end