FR: Parse screenName from ProvidedUserInfo

### Describe your environment

* Operating System version: Linux
* Firebase SDK version: v4.13.0
* Library version: v4.13.0
* Firebase Product: auth

### Describe the problem

#### Steps to reproduce:

When a user is logged in using GitHub as provider it returns the `screenName` aka `login` in GitHub when requesting this information via accounts.lookup in `ProvidedUserInfo` as described in https://cloud.google.com/identity-platform/docs/reference/rest/v1/ProviderUserInfo

#### Relevant Code:

Missing `ScreenName` field from `auth.UserInfo` struct to parse it into memory.

```diff
--- a/auth/user_mgt.go
+++ b/auth/user_mgt.go
@@ -59,6 +59,9 @@ type UserInfo struct {
// In UserRecord.UserInfo it will return the constant string "firebase".
ProviderID string `json:"providerId,omitempty"`
UID string `json:"rawId,omitempty"`
+ // ScreenName is the user's screen name at Twitter or login name at GitHub.
+ // Only populated in ProviderUserInfo[]
+ ScreenName string `json:"screenName,omitempty"`
}
```