cli: implement tailscale dns status (#13353)

Updates tailscale/tailscale#13326

This PR begins implementing a `tailscale dns` command group in the Tailscale CLI. It provides an initial implementation of `tailscale dns status` which dumps the state of the internal DNS forwarder.

Two new endpoints were added in LocalAPI to support the CLI functionality:

- `/netmap`: dumps a copy of the last received network map (because the CLI shouldn't have to listen to the ipn bus for a copy)
- `/dns-osconfig`: dumps the OS DNS configuration (this will be very handy for the UI clients as well, as they currently do not display this information)

My plan is to implement other subcommands mentioned in tailscale/tailscale#13326, such as `query`, in later PRs.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

ipn/ipnlocal/local.go

@@ -597,6 +597,15 @@ func (b *LocalBackend) SetComponentDebugLogging(component string, until time.Tim
 	return nil
 }
 
+// GetDNSOSConfig returns the base OS DNS configuration, as seen by the DNS manager.
+func (b *LocalBackend) GetDNSOSConfig() (dns.OSConfig, error) {
+	manager, ok := b.sys.DNSManager.GetOK()
+	if !ok {
+		return dns.OSConfig{}, errors.New("DNS manager not available")
+	}
+	return manager.GetBaseConfig()
+}
+
 // GetComponentDebugLogging gets the time that component's debug logging is
 // enabled until, or the zero time if component's time is not currently
 // enabled.