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>

ipn/ipnlocal/local.go

@@ -536,6 +536,8 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 		needsCaptiveDetection: make(chan bool),
 	}
 
+	sys.NoiseRoundTripper.Set(noiseRoundTripper{b})
+
 	nb := newNodeBackend(ctx, b.logf, b.sys.Bus.Get())
 	b.currentNodeAtomic.Store(nb)
 	nb.ready()
@@ -7274,6 +7276,15 @@ func (b *LocalBackend) DoNoiseRequest(req *http.Request) (*http.Response, error)
 	return cc.DoNoiseRequest(req)
 }
 
+// noiseRoundTripper adapts LocalBackend.DoNoiseRequest to http.RoundTripper.
+type noiseRoundTripper struct {
+	lb *LocalBackend
+}
+
+func (n noiseRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return n.lb.DoNoiseRequest(req)
+}
+
 // ActiveSSHConns returns the number of active SSH connections,
 // or 0 if SSH is not linked into the binary or available on the platform.
 func (b *LocalBackend) ActiveSSHConns() int {