Return a specific type from streaming_operation.

[obmarg]

AsyncWebsocketClient::streaming_operation currently returns an anonymous Stream type. This is fine for reading the results of a subscription, but there's a few things it can't support:

1. Stopping the subscription (arguably dropping the stream might do this, although not sure if it's handled very cleanly).
2. Checking why a stream ended.

Ideally we'd return a custom type that implements Stream but also:

1. Provides a stop function.
2. Stops the subscription cleanly on drop.
3. Provides a way to check whether the subscription stopped "cleanly" or because of some sort of connectivity error etc.

[obmarg]

There's a allow(dead_code) on the Message::Complete variant that can be removed once we have support for explicitly ending subscriptions.