net/rioconn: add PacketListener

In this commit, add PacketListener, a nettype.PacketListener whose
ListenPacket method returns a RIO-based UDPConn.

Updates tailscale/corp#8610

Signed-off-by: Nick Khyl <nickk@tailscale.com>

net/rioconn/packet_listener.go

@@ -0,0 +1,38 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build windows
+
+package rioconn
+
+import (
+	"context"
+	"net"
+
+	"tailscale.com/types/nettype"
+)
+
+// NewPacketListener returns a packet listener that uses Registered Input/Output (RIO)
+// API Extensions, if available, to provide high-performance UDP networking on Windows.
+// The specified options are applied to all connections created by the listener.
+// If RIO is not available, it returns an [ErrRIOUnavailable].
+func NewPacketListener(options ...UDPOption) (nettype.PacketListener, error) {
+	if err := Initialize(); err != nil {
+		return nil, err
+	}
+	return &PacketListener{options: options}, nil
+}
+
+// PacketListener is a [nettype.PacketListener] that uses RIO.
+type PacketListener struct {
+	options []UDPOption
+}
+
+// ListenPacket implements [nettype.PacketListener.ListenPacket].
+func (pl *PacketListener) ListenPacket(ctx context.Context, network, address string) (net.PacketConn, error) {
+	addr, err := net.ResolveUDPAddr(network, address)
+	if err != nil {
+		return nil, &net.OpError{Op: "listen", Net: network, Err: err}
+	}
+	return ListenUDP(network, addr, pl.options...)
+}