Add return event token when VEX upload

[kviset]

Added the ability to track the status of VEX processing:

Usage example:

token, err := client.VEX.Upload(context.TODO(),vexReq)
if err != nil {
       log.Fatal(err)
}
wait, err := client.Event.IsBeingProcessed(context.TODO(), dtrack.EventToken(token))
for {
        if wait {
                log.Info("VEX Processing")
                time.Sleep(time.Duration(5 * time.Second))
                wait, err = client.Event.IsBeingProcessed(context.TODO(), dtrack.EventToken(token))
        } else {
                log.Info("VEX Processing SUCCESS")
                break
        }
}

[owasp-dt-bot]

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