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/dialer/bytecounter.go

@@ -0,0 +1,93 @@
+package dialer
+
+import (
+	"context"
+	"net"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/bytecounter"
+)
+
+// ByteCounterDialer is a byte-counting-aware dialer. To perform byte counting, you
+// should make sure that you insert this dialer in the dialing chain.
+//
+// Bug
+//
+// This implementation cannot properly account for the bytes that are sent by
+// persistent connections, because they strick to the counters set when the
+// connection was established. This typically means we miss the bytes sent and
+// received when submitting a measurement. Such bytes are specifically not
+// see by the experiment specific byte counter.
+//
+// For this reason, this implementation may be heavily changed/removed.
+type ByteCounterDialer struct {
+	Dialer
+}
+
+// DialContext implements Dialer.DialContext
+func (d ByteCounterDialer) DialContext(
+	ctx context.Context, network, address string) (net.Conn, error) {
+	conn, err := d.Dialer.DialContext(ctx, network, address)
+	if err != nil {
+		return nil, err
+	}
+	exp := ContextExperimentByteCounter(ctx)
+	sess := ContextSessionByteCounter(ctx)
+	if exp == nil && sess == nil {
+		return conn, nil // no point in wrapping
+	}
+	return byteCounterConnWrapper{Conn: conn, exp: exp, sess: sess}, nil
+}
+
+type byteCounterSessionKey struct{}
+
+// ContextSessionByteCounter retrieves the session byte counter from the context
+func ContextSessionByteCounter(ctx context.Context) *bytecounter.Counter {
+	counter, _ := ctx.Value(byteCounterSessionKey{}).(*bytecounter.Counter)
+	return counter
+}
+
+// WithSessionByteCounter assigns the session byte counter to the context
+func WithSessionByteCounter(ctx context.Context, counter *bytecounter.Counter) context.Context {
+	return context.WithValue(ctx, byteCounterSessionKey{}, counter)
+}
+
+type byteCounterExperimentKey struct{}
+
+// ContextExperimentByteCounter retrieves the experiment byte counter from the context
+func ContextExperimentByteCounter(ctx context.Context) *bytecounter.Counter {
+	counter, _ := ctx.Value(byteCounterExperimentKey{}).(*bytecounter.Counter)
+	return counter
+}
+
+// WithExperimentByteCounter assigns the experiment byte counter to the context
+func WithExperimentByteCounter(ctx context.Context, counter *bytecounter.Counter) context.Context {
+	return context.WithValue(ctx, byteCounterExperimentKey{}, counter)
+}
+
+type byteCounterConnWrapper struct {
+	net.Conn
+	exp  *bytecounter.Counter
+	sess *bytecounter.Counter
+}
+
+func (c byteCounterConnWrapper) Read(p []byte) (int, error) {
+	count, err := c.Conn.Read(p)
+	if c.exp != nil {
+		c.exp.CountBytesReceived(count)
+	}
+	if c.sess != nil {
+		c.sess.CountBytesReceived(count)
+	}
+	return count, err
+}
+
+func (c byteCounterConnWrapper) Write(p []byte) (int, error) {
+	count, err := c.Conn.Write(p)
+	if c.exp != nil {
+		c.exp.CountBytesSent(count)
+	}
+	if c.sess != nil {
+		c.sess.CountBytesSent(count)
+	}
+	return count, err
+}