feature/featuretags, all: add ts_omit_acme to disable TLS cert support

I'd started to do this in the earlier ts_omit_server PR but
decided to split it into this separate PR.

Updates #17128

Change-Id: Ief8823a78d1f7bbb79e64a5cab30a7d0a5d6ff4b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/cert.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !ios && !android && !js
+//go:build !ios && !android && !js && !ts_omit_acme
 
 package localapi
 
@@ -14,6 +14,10 @@ import (
 	"tailscale.com/ipn/ipnlocal"
 )
 
+func init() {
+	Register("cert/", (*Handler).serveCert)
+}
+
 func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitWrite && !h.PermitCert {
 		http.Error(w, "cert access denied", http.StatusForbidden)