StreamSink impl async fn cancelled()

[modeckrus]

I use spawn to send something in stream via StreamSink, and I want to close this green thread when dart stream is closed. So I suggest to add method for StreamSink cancelled like in https://docs.rs/tokio-util/latest/tokio_util/sync/struct.CancellationToken.html

[welcome]

Hi! Thanks for opening your first issue here! 😄

[fzyzcjy]

Looks reasonable and feel free to PR for this new feature! Alternatively I may work on it later.

As a workaround before it is implemented, you can explicitly call a Rust function marking it is closed when that happens.