title: Firebase iOS SDK - proper way to do utm attribution for Push Notifications
body: <p>Have a client that is using &quot;session_start&quot; event to track conversions across 3 platforms on a Web+App property.</p>
  <p>For app links and deep links, this works perfectly. Clicking a universal link opens the app and fires the SESSION_START with utm attribution.</p>
  <p>For push notifications, this is ok on web and ok on Android (not sure how at this point, but that's another story). But on iOS, SESSION_START is not fired with attribution.</p>
  <p>Considering using custom NOTIFICATION_OPENED event, and attaching attribution parameters there.</p>
  <p>SDK does not support modifying or hijacking the SESSION_START event.</p>
  <p>What is the correct way to attach attribution to a push notification?</p>