feat(client): verify server certificates against the system root store (#5546)

transport.tls.trustedCaFile is installed as RootCAs, and omitting it sets
InsecureSkipVerify, so system-root verification cannot be selected directly.
Add an opt-in transport.tls.useSystemRoots that enables certificate
verification with RootCAs nil when no trusted CA file is set, allowing Go to
use system roots and native verification on macOS and Windows. Defaults are
unchanged.

pkg/transport/tls.go

@@ -126,6 +126,13 @@ func NewServerTLSConfig(certPath, keyPath, caPath string) (*tls.Config, error) {
 }
 
 func NewClientTLSConfig(certPath, keyPath, caPath, serverName string) (*tls.Config, error) {
+	return NewClientTLSConfigWithSystemRoots(certPath, keyPath, caPath, serverName, false)
+}
+
+// NewClientTLSConfigWithSystemRoots behaves like NewClientTLSConfig, except that when
+// caPath is empty and useSystemRoots is true, the server certificate is verified against
+// the host's root store.
+func NewClientTLSConfigWithSystemRoots(certPath, keyPath, caPath, serverName string, useSystemRoots bool) (*tls.Config, error) {
 	base := &tls.Config{}
 
 	if certPath != "" && keyPath != "" {
@@ -139,7 +146,8 @@ func NewClientTLSConfig(certPath, keyPath, caPath, serverName string) (*tls.Conf
 
 	base.ServerName = serverName
 
-	if caPath != "" {
+	switch {
+	case caPath != "":
 		pool, err := newCertPool(caPath)
 		if err != nil {
 			return nil, err
@@ -147,7 +155,11 @@ func NewClientTLSConfig(certPath, keyPath, caPath, serverName string) (*tls.Conf
 
 		base.RootCAs = pool
 		base.InsecureSkipVerify = false
-	} else {
+	case useSystemRoots:
+		// A nil RootCAs makes crypto/tls use the host root store, through the
+		// platform verifier on macOS and Windows.
+		base.InsecureSkipVerify = false
+	default:
 		base.InsecureSkipVerify = true
 	}