Log registrationID to simplify interactive node registration

Some clients such as Android make it hard to transfer the registrationID
to the server, its easier to get it from the server logs.

hscontrol/auth.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/juanfont/headscale/hscontrol/types/change"
+	"github.com/rs/zerolog/log"
 
 	"gorm.io/gorm"
 	"tailscale.com/tailcfg"
@@ -264,6 +265,7 @@ func (h *Headscale) handleRegisterInteractive(
 		nodeToRegister,
 	)
 
+	log.Info().Msgf("Starting node registration using key: %s", registrationId)
 	return &tailcfg.RegisterResponse{
 		AuthURL: h.authProvider.AuthURL(registrationId),
 	}, nil