Add reset stream API

[gjcairo]

Until now, a request handler had no way to abort the stream carrying a request.

This PR adds a reset operation to the response side:

• ResponseSender.reset(_:): abort before any response head is sent.
• Writer.reset(_:): abort a response that has already started.

Because a reset is inherently protocol-specific, reset hands the handler a protocol-discriminated NIOHTTPServer.StreamReset to switch over:

• .http2 → sends an HTTP/2 RST_STREAM with a chosen HTTP2ErrorCode.
• .http3 → sends an HTTP/3 RESET_STREAM with a chosen QUIC application error code.
• .unavailable → HTTP/1.1, which has no per-stream reset.

reset consumes the sender/writer, so users are prevented from writing anything further once the stream has been reset.

An example scenario using CONNECT:

  let writer = try await responseSender.send(.init(status: .ok))   // e.g. CONNECT tunnel established
  // … the tunnelled connection fails …
  try writer.reset { streamReset in
      switch consume streamReset {
      case .http2(let h2): 
        h2.reset(code: .connectError)
      case .http3(let h3): 
        h3.reset(code: 0x010f)   // H3_CONNECT_ERROR
      case .unavailable:
        break
      }
  }

[ehaydenr]

Related apple/swift-nio-http3#14

[aryan-25]

nit:

Suggested change

	// The `HTTP2FramePayloadToHTTPServerCodec` on the stream channel translates this event into an `RST_STREAM`
	// The `HTTP2FramePayloadToHTTPServerCodec` on the stream channel translates this event into a `RST_STREAM`