Add timeline read markers API (#11762)

Fix #4093

app/serializers/rest/marker_serializer.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class REST::MarkerSerializer < ActiveModel::Serializer
+  attributes :last_read_id, :version, :updated_at
+
+  def last_read_id
+    object.last_read_id.to_s
+  end
+
+  def version
+    object.lock_version
+  end
+end