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/localclient/local-client.go

@@ -19,6 +19,7 @@ type LocalClient interface {
 	WatchIPNBus(ctx context.Context, mask ipn.NotifyWatchOpt) (IPNBusWatcher, error)
 	SetServeConfig(context.Context, *ipn.ServeConfig) error
 	EditPrefs(ctx context.Context, mp *ipn.MaskedPrefs) (*ipn.Prefs, error)
+	CertDomains(ctx context.Context) ([]string, error)
 	CertIssuer
 }
 
@@ -57,3 +58,7 @@ func (lc *localClient) WatchIPNBus(ctx context.Context, mask ipn.NotifyWatchOpt)
 func (lc *localClient) CertPair(ctx context.Context, domain string) ([]byte, []byte, error) {
 	return lc.lc.CertPair(ctx, domain)
 }
+
+func (lc *localClient) CertDomains(ctx context.Context) ([]string, error) {
+	return lc.lc.CertDomains(ctx)
+}