DEV: Add hooks to allow overriding notify_user behavior (#23850)

Adds new plugin registry `:post_action_notify_user_handlers` and more!

lib/guardian/post_guardian.rb

@@ -58,7 +58,19 @@ module PostGuardian
 
         if action_key == :notify_user &&
              !@user.in_any_groups?(SiteSetting.personal_message_enabled_groups_map)
-          return false
+          # The modifier below is used to add additional permissions for notifying users.
+          # In core the only method of notifying a user is personal messages so we check if the
+          # user can PM. Plugins can extend the behavior of how users are notifier via `notify_user`
+          # post action, and this allows extension for that use case.
+          can_notify = false
+          can_notify =
+            DiscoursePluginRegistry.apply_modifier(
+              :post_guardian_can_notify_user,
+              can_notify,
+              self,
+              post,
+            )
+          return can_notify
         end
 
         # we allow flagging for trust level 1 and higher