http3: remove dependency on quic internal packages (#5256)

* Remove http3 dependency on quic internal packages

Remove the dependency on internal/protocol from the http3 package. This
makes it possible for a forked http3 to use the mainline quic-go
package.

* Address review comments

* Fix syntax

* Use broader pattern for http3 directory

* Copy internal/testdata

* Replace perspective with bool

* clone the supported version slice

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

http3/internal/testdata/cert.go

@@ -0,0 +1,56 @@
+package testdata
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+	"os"
+	"path"
+	"runtime"
+)
+
+var certPath string
+
+func init() {
+	_, filename, _, ok := runtime.Caller(0)
+	if !ok {
+		panic("Failed to get current frame")
+	}
+
+	certPath = path.Dir(filename)
+}
+
+// GetCertificatePaths returns the paths to certificate and key
+func GetCertificatePaths() (string, string) {
+	return path.Join(certPath, "cert.pem"), path.Join(certPath, "priv.key")
+}
+
+// GetTLSConfig returns a tls config for quic.clemente.io
+func GetTLSConfig() *tls.Config {
+	cert, err := tls.LoadX509KeyPair(GetCertificatePaths())
+	if err != nil {
+		panic(err)
+	}
+	return &tls.Config{
+		MinVersion:   tls.VersionTLS13,
+		Certificates: []tls.Certificate{cert},
+	}
+}
+
+// AddRootCA adds the root CA certificate to a cert pool
+func AddRootCA(certPool *x509.CertPool) {
+	caCertPath := path.Join(certPath, "ca.pem")
+	caCertRaw, err := os.ReadFile(caCertPath)
+	if err != nil {
+		panic(err)
+	}
+	if ok := certPool.AppendCertsFromPEM(caCertRaw); !ok {
+		panic("Could not add root ceritificate to pool.")
+	}
+}
+
+// GetRootCA returns an x509.CertPool containing (only) the CA certificate
+func GetRootCA() *x509.CertPool {
+	pool := x509.NewCertPool()
+	AddRootCA(pool)
+	return pool
+}