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/httptransport.go

@@ -0,0 +1,47 @@
+// Package httptransport contains HTTP transport extensions.
+package httptransport
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+	"net/http"
+
+	"github.com/lucas-clemente/quic-go"
+)
+
+// Config contains the configuration required for constructing an HTTP transport
+type Config struct {
+	Dialer     Dialer
+	QUICDialer QUICDialer
+	TLSDialer  TLSDialer
+	TLSConfig  *tls.Config
+}
+
+// Dialer is the definition of dialer assumed by this package.
+type Dialer interface {
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// TLSDialer is the definition of a TLS dialer assumed by this package.
+type TLSDialer interface {
+	DialTLSContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// QUICDialer is the definition of dialer for QUIC assumed by this package.
+type QUICDialer interface {
+	Dial(network, addr string, tlsCfg *tls.Config, cfg *quic.Config) (quic.EarlySession, error)
+}
+
+// RoundTripper is the definition of http.RoundTripper used by this package.
+type RoundTripper interface {
+	RoundTrip(req *http.Request) (*http.Response, error)
+	CloseIdleConnections()
+}
+
+// Resolver is the interface we expect from a resolver
+type Resolver interface {
+	LookupHost(ctx context.Context, hostname string) (addrs []string, err error)
+	Network() string
+	Address() string
+}