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/ipnlocal/cert_disabled.go

@@ -0,0 +1,45 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build js || ts_omit_acme
+
+package ipnlocal
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net/http"
+	"time"
+)
+
+func init() {
+	RegisterC2N("GET /tls-cert-status", handleC2NTLSCertStatusDisabled)
+}
+
+var errNoCerts = errors.New("cert support not compiled in this build")
+
+type TLSCertKeyPair struct {
+	CertPEM, KeyPEM []byte
+}
+
+func (b *LocalBackend) GetCertPEM(ctx context.Context, domain string) (*TLSCertKeyPair, error) {
+	return nil, errNoCerts
+}
+
+var errCertExpired = errors.New("cert expired")
+
+type certStore interface{}
+
+func getCertPEMCached(cs certStore, domain string, now time.Time) (p *TLSCertKeyPair, err error) {
+	return nil, errNoCerts
+}
+
+func (b *LocalBackend) getCertStore() (certStore, error) {
+	return nil, errNoCerts
+}
+
+func handleC2NTLSCertStatusDisabled(b *LocalBackend, w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	io.WriteString(w, `{"Missing":true}`) // a minimal tailcfg.C2NTLSCertInfo
+}