ipn/localapi: move cert fetching code to localapi, cache, add cert subcommand

Updates #1235

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

ipn/localapi/disabled_stubs.go

@@ -0,0 +1,17 @@
+// 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.
+
+//go:build ios || android
+// +build ios android
+
+package localapi
+
+import (
+	"net/http"
+	"runtime"
+)
+
+func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
+	http.Error(w, "disabled on "+runtime.GOOS, http.StatusNotFound)
+}