ipn/ipnlocal,tsd: add NoiseRoundTripper to tsd.Sys

Adds a new NoiseRoundTripper field to tsd.Sys
to expose an http.RoundTripper to make requests
over the control plane Noise connection.

This will be used in PAM use cases soon.

Updates tailscale/corp#41800

Signed-off-by: Adriano Sela Aviles <adriano@tailscale.com>

tsd/tsd.go

@@ -20,6 +20,7 @@ package tsd
 import (
 	"crypto/x509"
 	"fmt"
+	"net/http"
 	"reflect"
 
 	"tailscale.com/control/controlknobs"
@@ -64,6 +65,10 @@ type System struct {
 	PolicyClient   SubSystem[policyclient.Client]
 	HealthTracker  SubSystem[*health.Tracker]
 
+	// NoiseRoundTripper, if set, provides an http.RoundTripper that
+	// sends requests over the control plane Noise connection.
+	NoiseRoundTripper SubSystem[http.RoundTripper]
+
 	// ExtraRootCAs, if non-nil, specifies additional trusted root CAs
 	// beyond the system roots. On Android, this includes user-installed
 	// CA certificates that Go's crypto/x509 does not see.