integration: add tool for renewing test certs (#4742)

Add a utility for updating the integration test certificates. It takes
three file paths: the existing certificate, the CA certificate, and the
CA key. It will update the NotBefore and NotAfter timestamps and the
certificate signature, overwriting the existing certificate.

Example usage:

  cd integration/tpl/files
  go run renew-cert.go trusted.pem ca.pem ca-key.pem

integration/tpl/files/renew-cert.go

@@ -0,0 +1,74 @@
+//go:build ignore
+
+package main
+
+import (
+	"crypto/rand"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"log"
+	"os"
+	"time"
+)
+
+func main() {
+	if len(os.Args) < 4 {
+		fmt.Fprintln(os.Stderr, "usage: go run renew-cert.go <cert-path> <ca-path> <ca-key-path>")
+		os.Exit(2)
+	}
+
+	certPath := os.Args[1]
+	b, err := os.ReadFile(certPath)
+	if err != nil {
+		log.Fatalln(err)
+	}
+	block, _ := pem.Decode(b)
+	tpl, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	// Parse the CA certificate and private key.
+	b, err = os.ReadFile(os.Args[2])
+	if err != nil {
+		log.Fatalln(err)
+	}
+	block, _ = pem.Decode(b)
+	caCert, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		log.Fatalln(err)
+	}
+	b, err = os.ReadFile(os.Args[3])
+	if err != nil {
+		log.Fatalln(err)
+	}
+	block, _ = pem.Decode(b)
+	caKey, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	if tpl.CheckSignatureFrom(caCert) != nil {
+		log.Fatalln("error: cert was not issued by the provided CA")
+	}
+
+	now := time.Now()
+	tpl.NotBefore = now
+	tpl.NotAfter = now.Add(3650 * 24 * time.Hour)
+
+	// Issue the new certificate.
+	b, err = x509.CreateCertificate(rand.Reader, tpl, caCert, tpl.PublicKey, caKey)
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	// Write the new certificate to a PEM file.
+	b = pem.EncodeToMemory(&pem.Block{
+		Type:  "CERTIFICATE",
+		Bytes: b,
+	})
+	if err := os.WriteFile(certPath, b, 0644); err != nil {
+		log.Fatalln(err)
+	}
+}