tsnet: add HTTP client method to tsnet.Server (#6669)

This allows tsnet services to make requests to other services in the
tailnet with the tsnet service identity instead of the identity of the
host machine. This also enables tsnet services to make requests to other
tailnet services without having to have the host machine join the
tailnet.

Signed-off-by: Xe Iaso <xe@tailscale.com>

Signed-off-by: Xe Iaso <xe@tailscale.com>

tsnet/example/tsnet-http-client/tsnet-http-client.go

@@ -0,0 +1,45 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The tshello server demonstrates how to use Tailscale as a library.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+
+	"tailscale.com/tsnet"
+)
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprintf(os.Stderr, "Usage: %s <url in tailnet>\n", filepath.Base(os.Args[0]))
+		os.Exit(2)
+	}
+	flag.Parse()
+
+	if flag.NArg() != 1 {
+		flag.Usage()
+	}
+	tailnetURL := flag.Arg(0)
+	
+	s := new(tsnet.Server)
+	defer s.Close()
+
+	if err := s.Start(); err != nil {
+		log.Fatal(err)
+	}
+
+	cli := s.HTTPClient()
+
+	resp, err := cli.Get(tailnetURL)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	resp.Write(os.Stdout)
+}