* GH-3959: MqttConFailedEvent for normal disconnect
Fixes https://github.com/spring-projects/spring-integration/issues/3959
The `MqttCallback.disconnected(MqttDisconnectResponse)` in Paho v5 client is also
called when server initiates a disconnection.
In this case that `MqttDisconnectResponse` does not have a `cause` value
* Modify `MqttConnectionFailedEvent` to make a `cause` property optional
* Fix `Mqttv5PahoMessageDrivenChannelAdapter` & `Mqttv5PahoMessageHandler`
to not check for `cause`, but emit an `MqttConnectionFailedEvent` for any `disconnected()` calls
Unfortunately current Paho v3 client does not call `connectionLost()` for normal disconnections
and we cannot react for this callback with an `MqttConnectionFailedEvent`
**Cherry-pick to `5.5.x`**
* Fix language in doc
Co-authored-by: Gary Russell <grussell@vmware.com>
Co-authored-by: Gary Russell <grussell@vmware.com>
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