FEATURE: Add 'New users only' option to user_updated trigger (#26648)

This commit adds a new option to the `user_updated` trigger of the automation plugin to only trigger an automation for new users that join after the automation is enabled.

Internal topic: t/125829/9.

plugins/automation/app/models/discourse_automation/automation.rb

@@ -20,6 +20,10 @@ module DiscourseAutomation
     validates :script, presence: true
     validate :validate_trigger_fields
 
+    after_destroy do |automation|
+      UserCustomField.where(name: automation.new_user_custom_field_name).destroy_all
+    end
+
     attr_accessor :running_in_background
 
     def running_in_background!
@@ -163,6 +167,10 @@ module DiscourseAutomation
       scriptable&.on_reset&.call(self)
     end
 
+    def new_user_custom_field_name
+      "automation_#{self.id}_new_user"
+    end
+
     private
 
     def validate_trigger_fields