How to know when the ManagedMqttClient is subscribed?

### Describe your question
We have a use case where I need to know when the `ManagedMqttClient` is subscribed to a topic after calling `SubscribeAsync`. I have several workarounds for the problem, such as calling `SubscribeAsync` on the `InternalClient` or waiting for a short period before continuing. But I was wondering if there is a proper way for knowing when the `ManagedMqttClient` has subscribed to the topic?

We need to know when we are subscribed before starting other tasks. Otherwise we end up missing messages because the `ManagedMqttClient` implements the operation asynchronously. It can easily be reproduced if you call `PublishAsync` directly after `SubscribeAsync` on the same topic - the message will likely be published before you are subscribed.

### Which project is your question related to?
- ManagedClient

Thanks!