Enumerates SkAdNetwork user types

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v26/enums/sk_ad_network_user_type.proto)

| Enums ||
|---|---|
| ## `NEW_INSTALLER` | The user installed the app for the first time. |
| ## `REINSTALLER` | The user has previously installed the app. |
| ## `UNAVAILABLE` | The value was not present in the postback or we do not have this data for other reasons. |
| ## `UNKNOWN` | The value is unknown in this version. |
| ## `UNSPECIFIED` | Not specified. |