Add administrative webhooks (#18510)

* Add administrative webhooks

* Fix error when webhook is deleted before delivery worker runs

app/serializers/rest/admin/webhook_event_serializer.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class REST::Admin::WebhookEventSerializer < ActiveModel::Serializer
+  def self.serializer_for(model, options)
+    case model.class.name
+    when 'Account'
+      REST::Admin::AccountSerializer
+    when 'Report'
+      REST::Admin::ReportSerializer
+    else
+      super
+    end
+  end
+
+  attributes :event, :created_at
+
+  has_one :virtual_object, key: :object
+
+  def virtual_object
+    object.object
+  end
+
+  def event
+    object.type
+  end
+end