Detect client disconnection with `body::Sender`

[losfair]

Describe the solution you'd like

I'm using hyper::body::Sender to send an SSE stream to the client. For each connection there is an associated Tokio task that subscribes to a distributed message queue, and I would like to stop the task on client disconnection. It seems that currently there isn't a method on Sender that allows to detect disconnection without attempting to send a message.

I think an API like this would work:

impl Sender {
  pub async fn wait_disconnect(&mut self) -> Result<()>;
}

Describe alternatives you've considered

1. Send heartbeat messages to the client periodically and stop the task on failure.
2. Periodically call poll_ready.

I'm already doing 1 anyway, but this adds latency between the client closing the connection and the server stopping the task, even when the connection is gracefully shut down.

[nwtgck]

My project using hyper also needs this feature.