appc,feature: add the start of new conn25 app connector

When peers request an IP address mapping to be stored, the connector
stores it in memory.

Fixes tailscale/corp#34251
Signed-off-by: Fran Bull <fran@tailscale.com>

feature/condregister/maybe_conn25.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_conn25
+
+package condregister
+
+import _ "tailscale.com/feature/conn25"