net/netns: add Windows support for bind-to-interface-by-route

This is implemented via GetBestInterfaceEx. Should we encounter errors
or fail to resolve a valid, non-Tailscale interface, we fall back to
returning the index for the default interface instead.

Fixes #12551

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/netns/netns_dw.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build darwin || windows
+
+package netns
+
+import (
+	"net"
+	"net/netip"
+)
+
+func parseAddress(address string) (addr netip.Addr, err error) {
+	host, _, err := net.SplitHostPort(address)
+	if err != nil {
+		// error means the string didn't contain a port number, so use the string directly
+		host = address
+	}
+
+	return netip.ParseAddr(host)
+}