tsnet: add support for Services

This change allows tsnet nodes to act as Service hosts by adding a new
function, tsnet.Server.ListenService. Invoking this function will
advertise the node as a host for the Service and create a listener to
receive traffic for the Service.

Fixes #17697
Fixes tailscale/corp#27200
Signed-off-by: Harry Harpham <harry@tailscale.com>

ipn/ipnlocal/local.go

@@ -399,6 +399,10 @@ type LocalBackend struct {
 	// hardwareAttested is whether backend should use a hardware-backed key to
 	// bind the node identity to this device.
 	hardwareAttested atomic.Bool
+
+	// getCertForTest is used to retrieve TLS certificates in tests.
+	// See [LocalBackend.ConfigureCertsForTest].
+	getCertForTest func(hostname string) (*TLSCertKeyPair, error)
 }
 
 // SetHardwareAttested enables hardware attestation key signatures in map