tsnet,wgengine/netstack: add ListenPacket and tests

This adds a new ListenPacket function on tsnet.Server
which acts mostly like `net.ListenPacket`.

Unlike `Server.Listen`, this requires listening on a
specific IP and does not automatically listen on both
V4 and V6 addresses of the Server when the IP is unspecified.

To test this, it also adds UDP support to tsdial.Dialer.UserDial
and plumbs it through the localapi. Then an associated test
to make sure the UDP functionality works from both sides.

Updates #12182

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/localapi/localapi.go

@@ -6,6 +6,7 @@ package localapi
 
 import (
 	"bytes"
+	"cmp"
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
@@ -1939,8 +1940,10 @@ func (h *Handler) serveDial(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	network := cmp.Or(r.Header.Get("Dial-Network"), "tcp")
+
 	addr := net.JoinHostPort(hostStr, portStr)
-	outConn, err := h.b.Dialer().UserDial(r.Context(), "tcp", addr)
+	outConn, err := h.b.Dialer().UserDial(r.Context(), network, addr)
 	if err != nil {
 		http.Error(w, "dial failure: "+err.Error(), http.StatusBadGateway)
 		return