Update AppDistribution [@type] (#1088)

This is a short followup to update the App Distribution @type.

[More info here](c4f63c7)

```
type.googleapis.com/google.events.firebase.firebasealerts.v1.AppDistroNewTesterIosDevicePayload
```

src/v2/providers/alerts/appDistribution.ts

@@ -7,7 +7,7 @@ import { FirebaseAlertData, getEndpointAnnotation } from './alerts';
  * Payload is wrapped inside a FirebaseAlertData object.
  */
 export interface NewTesterDevicePayload {
-  ['@type']: 'com.google.firebase.firebasealerts.NewTesterDevicePayload';
+  ['@type']: 'type.googleapis.com/google.events.firebase.firebasealerts.v1.AppDistroNewTesterIosDevicePayload';
   testerName: string;
   testerEmail: string;
   testerDeviceModelName: string;