refactor: move base http3 transport into netxlite (#412)

This diff is part of https://github.com/ooni/probe/issues/1505.

You will notice that I have not adapted all the (great) tests we had
previously. They should live at another layer, and namely the one that
deals with performing measurements.

When I'm refactoring such a layer I'll ensure those tests that I have
not adapted here are reintroduced into the tree.

internal/netxlite/http3.go

@@ -0,0 +1,54 @@
+package netxlite
+
+import (
+	"context"
+	"crypto/tls"
+	"net/http"
+
+	"github.com/lucas-clemente/quic-go"
+	"github.com/lucas-clemente/quic-go/http3"
+)
+
+// http3Dialer adapts a QUICContextDialer to work with
+// an http3.RoundTripper. This is necessary because the
+// http3.RoundTripper does not support DialContext.
+type http3Dialer struct {
+	Dialer QUICContextDialer
+}
+
+// dial is like QUICContextDialer.DialContext but without context.
+func (d *http3Dialer) dial(network, address string, tlsConfig *tls.Config,
+	quicConfig *quic.Config) (quic.EarlySession, error) {
+	return d.Dialer.DialContext(
+		context.Background(), network, address, tlsConfig, quicConfig)
+}
+
+// http3Transport is an HTTPTransport using the http3 protocol.
+type http3Transport struct {
+	child *http3.RoundTripper
+}
+
+var _ HTTPTransport = &http3Transport{}
+
+// RoundTrip implements HTTPTransport.RoundTrip.
+func (txp *http3Transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	return txp.child.RoundTrip(req)
+}
+
+// CloseIdleConnections implements HTTPTransport.CloseIdleConnections.
+func (txp *http3Transport) CloseIdleConnections() {
+	txp.child.Close()
+}
+
+// NewHTTP3Transport creates a new HTTPTransport using http3. The
+// dialer argument MUST NOT be nil. If the tlsConfig argument is nil,
+// then the code will use the default TLS configuration.
+func NewHTTP3Transport(
+	dialer QUICContextDialer, tlsConfig *tls.Config) HTTPTransport {
+	return &http3Transport{
+		child: &http3.RoundTripper{
+			Dial:            (&http3Dialer{dialer}).dial,
+			TLSClientConfig: tlsConfig,
+		},
+	}
+}