client/local, ipn/localapi, all: add CertDomains and DNSConfig accessors

Add two narrow LocalAPI accessors so callers don't have to subscribe to
the IPN bus and pull a full *netmap.NetworkMap just to read DNS-shaped
fields:

  - GET /localapi/v0/cert-domains returns DNS.CertDomains.
  - GET /localapi/v0/dns-config returns the full tailcfg.DNSConfig.

Migrate in-tree callers off the netmap-on-the-bus pattern:

  - kube/certs.waitForCertDomain still wakes on the IPN bus but now
    queries CertDomains via LocalClient.CertDomains rather than
    reading n.NetMap.DNS.CertDomains. The kube LocalClient interface
    and FakeLocalClient gain a CertDomains method.
  - cmd/tailscale dns status calls LocalClient.DNSConfig directly
    instead of opening a NotifyInitialNetMap watcher.
  - cmd/tailscale configure kubeconfig switches from a netmap watcher
    + serviceDNSRecordFromNetMap to LocalClient.DNSConfig +
    serviceDNSRecordFromDNSConfig.

This is part of a series moving callers away from depending on the
netmap traveling on the IPN bus, so the bus payload can shrink in a
later change.

Updates #12542

Change-Id: Ie10204e141d085fbac183b4cfe497226b670ad6c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

kube/certs/certs_test.go

@@ -201,18 +201,12 @@ func TestEnsureCertLoops(t *testing.T) {
 
 			notifyChan := make(chan ipn.Notify)
 			go func() {
+				// Drive waitForCertDomain by sending notifications
+				// with empty netmaps as wake-up triggers; the cert
+				// manager queries CertDomains via the local
+				// client and not by reading the bus payload.
 				for {
-					notifyChan <- ipn.Notify{
-						NetMap: &netmap.NetworkMap{
-							DNS: tailcfg.DNSConfig{
-								CertDomains: []string{
-									"my-app.tailnetxyz.ts.net",
-									"my-other-app.tailnetxyz.ts.net",
-									"my-apiserver.tailnetxyz.ts.net",
-								},
-							},
-						},
-					}
+					notifyChan <- ipn.Notify{NetMap: &netmap.NetworkMap{}}
 				}
 			}()
 			cm := &CertManager{
@@ -220,6 +214,11 @@ func TestEnsureCertLoops(t *testing.T) {
 					FakeIPNBusWatcher: localclient.FakeIPNBusWatcher{
 						NotifyChan: notifyChan,
 					},
+					CertDomainsResult: []string{
+						"my-app.tailnetxyz.ts.net",
+						"my-other-app.tailnetxyz.ts.net",
+						"my-apiserver.tailnetxyz.ts.net",
+					},
 				},
 				logf:      log.Printf,
 				certLoops: make(map[string]context.CancelFunc),