fix: add Send bound to Streaming wrapper (#63)

useful for moving ownership to the spawn closure,

```
let mut response_stream = client.append_session(request).await?;

    let response_consumer = tokio::spawn(async move {
        while let Some(n) = response_stream.next().await {
            if let Err(e) = n {
                error!(?e, "receiver error")
            }
        }
    });
```

src/service.rs

@@ -193,13 +193,12 @@ impl<S: StreamingResponse> futures::Stream for ServiceStreamingResponse<S> {
     }
 }
 
-/// Wrapper around `ServiceStreamingResponse` to expose publically.
-pub struct Streaming<R>(Box<dyn Unpin + futures::Stream<Item = Result<R, ClientError>>>);
+pub struct Streaming<R>(Box<dyn Unpin + Send + futures::Stream<Item = Result<R, ClientError>>>);
 
 impl<R> Streaming<R> {
     pub(crate) fn new<S>(s: ServiceStreamingResponse<S>) -> Self
     where
-        S: StreamingResponse<ResponseItem = R> + 'static,
+        S: StreamingResponse<ResponseItem = R> + Send + 'static,
     {
         Self(Box::new(s))
     }