ipn/ipnlocal: add c2n method to check on TLS cert fetch status

So the control plane can delete TXT records more aggressively
after client's done with ACME fetch.

Updates tailscale/corp#15848

Change-Id: I4f1140305bee11ee3eee93d4fec3aef2bd6c5a7e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/cert_js.go

@@ -6,6 +6,7 @@ package ipnlocal
 import (
 	"context"
 	"errors"
+	"time"
 )
 
 type TLSCertKeyPair struct {
@@ -15,3 +16,15 @@ type TLSCertKeyPair struct {
 func (b *LocalBackend) GetCertPEM(ctx context.Context, domain string) (*TLSCertKeyPair, error) {
 	return nil, errors.New("not implemented for js/wasm")
 }
+
+var errCertExpired = errors.New("cert expired")
+
+type certStore interface{}
+
+func getCertPEMCached(cs certStore, domain string, now time.Time) (p *TLSCertKeyPair, err error) {
+	return nil, errors.New("not implemented for js/wasm")
+}
+
+func (b *LocalBackend) getCertStore() (certStore, error) {
+	return nil, errors.New("not implemented for js/wasm")
+}