ipn/ipnlocal: add peerapi endpoint for relay server endpoint allocation

Relay server initialization is performed as part of handling peerapi
endpoint allocation requests in similar fashion to SSH server
init. The relay server is not supported on iOS for now.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/udprelay/server_default.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !ios
+
 // Package udprelay contains constructs for relaying Disco and WireGuard packets
 // between Tailscale clients over UDP. This package is currently considered
 // experimental.

net/udprelay/server_default_test.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !ios
+
 package udprelay
 
 import (

net/udprelay/server_ios.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ios
+
+package udprelay
+
+type ServerEndpoint struct{}