net/tcpinfo: add package to allow fetching TCP information

This package contains platform-independent abstractions for fetching
information about an open TCP connection.

Updates #8413

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I236657b1060d7e6a45efc7a2f6aacf474547a2fe

net/tcpinfo/tcpinfo_other.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux && !darwin
+
+package tcpinfo
+
+import (
+	"net"
+	"time"
+)
+
+func rttImpl(conn *net.TCPConn) (time.Duration, error) {
+	return 0, ErrUnimplemented
+}