FEATURE: auto_action_type field for flags (#29306)

Allow admins to specify if the flag should be `auto_action_type`. If yes, then when an admin flags a post,  it is automatically actioned.

Meta: https://meta.discourse.org/t/allow-creation-of-custom-flags-which-auto-hide-content-similar-to-spam-and-inapproriate/329894

app/serializers/flag_serializer.rb

@@ -12,7 +12,8 @@ class FlagSerializer < ApplicationSerializer
              :enabled,
              :is_flag,
              :applies_to,
-             :is_used
+             :is_used,
+             :auto_action_type
 
   def i18n_prefix
     "#{@options[:target] || "post_action"}_types.#{object.name_key}"