ipn/ipnlocal, wgengine/netstack: start handling ports for future serving

Updates tailscale/corp#7515

Change-Id: I966e936e72a2ee99be8d0f5f16872b48cc150258
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/cert_js.go

@@ -0,0 +1,18 @@
+// 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 (
+	"context"
+	"errors"
+)
+
+type TLSCertKeyPair struct {
+	CertPEM, KeyPEM []byte
+}
+
+func (b *LocalBackend) GetCertPEM(ctx context.Context, domain string) (*TLSCertKeyPair, error) {
+	return nil, errors.New("not implemented for js/wasm")
+}