tailcfg, control/controlclient, ipn/ipnlocal: add c2n (control-to-node) system

This lets the control plane can make HTTP requests to nodes.

Then we can use this for future things rather than slapping more stuff
into MapResponse, etc.

Change-Id: Ic802078c50d33653ae1f79d1e5257e7ade4408fd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2022 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.
+
+package ipnlocal
+
+import (
+	"io"
+	"net/http"
+)
+
+func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
+	switch r.URL.Path {
+	case "/echo":
+		// Test handler.
+		body, _ := io.ReadAll(r.Body)
+		w.Write(body)
+	default:
+		http.Error(w, "unknown c2n path", http.StatusBadRequest)
+	}
+}