chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/netx/httptransport/http3transport.go

@@ -0,0 +1,43 @@
+package httptransport
+
+import (
+	"context"
+	"crypto/tls"
+	"net/http"
+
+	"github.com/lucas-clemente/quic-go"
+	"github.com/lucas-clemente/quic-go/http3"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/quicdialer"
+)
+
+// QUICWrapperDialer is a QUICDialer that wraps a ContextDialer
+// This is necessary because the http3 RoundTripper does not support a DialContext method.
+type QUICWrapperDialer struct {
+	Dialer quicdialer.ContextDialer
+}
+
+// Dial implements QUICDialer.Dial
+func (d QUICWrapperDialer) Dial(network, host string, tlsCfg *tls.Config, cfg *quic.Config) (quic.EarlySession, error) {
+	return d.Dialer.DialContext(context.Background(), network, host, tlsCfg, cfg)
+}
+
+// HTTP3Transport is a httptransport.RoundTripper using the http3 protocol.
+type HTTP3Transport struct {
+	http3.RoundTripper
+}
+
+// CloseIdleConnections closes all the connections opened by this transport.
+func (t *HTTP3Transport) CloseIdleConnections() {
+	t.RoundTripper.Close()
+}
+
+// NewHTTP3Transport creates a new HTTP3Transport instance.
+func NewHTTP3Transport(config Config) RoundTripper {
+	txp := &HTTP3Transport{}
+	txp.QuicConfig = &quic.Config{}
+	txp.TLSClientConfig = config.TLSConfig
+	txp.Dial = config.QUICDialer.Dial
+	return txp
+}
+
+var _ RoundTripper = &http.Transport{}