Recover the session status from somewhere else

### Describe the feature request

We want to implement a cluster with high availability based on MQTTnet. Sometimes we need to close a pod and migrate all the client sessions to another pod. However, we cannot restore client sessions in the `MqttServer` class.

### Which project is your feature request related to?

- Server

### Describe the solution you'd like
I find the class `PreparingSessionEventArgs` which seems related to this feature, but the class is not used. Look forward to the finalizing of this feature.

### Describe alternatives you've considered

Do the session management on my own.