SignalR Java client: disposing Completable returned by HubConnection.start() does nothing

### Describe the bug
It is not possible to prevent `HubConnection` from starting with dispose call on `Completable` returned by `HubConnection.start()`.
[By looking at the code there is no `doOnDispose` or alike](https://github.com/dotnet/aspnetcore/blob/a9986c33a51f5ceda5854cbfc8969f4c9079caf7/src/SignalR/clients/java/signalr/core/src/main/java/com/microsoft/signalr/HubConnection.java#L225)

If one is unaware of this, it could lead to unwanted connections.

### To Reproduce
```java
HubConnection hubConnection= HubConnectionBuilder.create(...)...;
hubConnection
.start()
.subscribe(connection->/*do something*/)
.dispose(); // this does nothing, connection is created
```

### Further technical details
- Client version: 6.0.0-preview.7.21378.6