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/shaping_enabled.go

@@ -0,0 +1,40 @@
+// +build shaping
+
+package dialer
+
+import (
+	"context"
+	"net"
+	"time"
+)
+
+// ShapingDialer ensures we don't use too much bandwidth
+// when using integration tests at GitHub. To select
+// the implementation with shaping use `-tags shaping`.
+type ShapingDialer struct {
+	Dialer
+}
+
+// DialContext implements Dialer.DialContext
+func (d ShapingDialer) DialContext(
+	ctx context.Context, network, address string) (net.Conn, error) {
+	conn, err := d.Dialer.DialContext(ctx, network, address)
+	if err != nil {
+		return nil, err
+	}
+	return &shapingConn{Conn: conn}, nil
+}
+
+type shapingConn struct {
+	net.Conn
+}
+
+func (c shapingConn) Read(p []byte) (int, error) {
+	time.Sleep(100 * time.Millisecond)
+	return c.Conn.Read(p)
+}
+
+func (c shapingConn) Write(p []byte) (int, error) {
+	time.Sleep(100 * time.Millisecond)
+	return c.Conn.Write(p)
+}