Request client setting to allow RST instead of FIN on request cancelation.

[BlackRabbitCoder]

Problem

When a client request future is dropped/cancelled mid-send (e.g., partial body),
hyper-util sends FIN via poll_shutdown(). This tells the server "I'm done sending"
rather than "abort/discard."

Why RST would be better

• FIN + truncated body can cause server to wait for remaining data (timeout)
• Server might attempt to process incomplete request
• RST semantics ("abort") match the intent of cancellation

Current behavior

1. Request future dropped
2. Connection detects cancellation via poll_canceled()
3. poll_shutdown() called → FIN sent
4. Server receives incomplete request + graceful close

Suggested behavior

Option to send RST instead of FIN when a request is cancelled mid-flight.
Could be a pool config option like rst_on_cancel(true).

Workaround

Currently requires setting SO_LINGER=0 on the socket in a custom connector somehow just before connection is dropped.