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/legacy/netx/dialid/dialid.go

@@ -0,0 +1,24 @@
+package dialid
+
+import (
+	"context"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/atomicx"
+)
+
+type contextkey struct{}
+
+var id = atomicx.NewInt64()
+
+// WithDialID returns a copy of ctx with DialID
+func WithDialID(ctx context.Context) context.Context {
+	return context.WithValue(
+		ctx, contextkey{}, id.Add(1),
+	)
+}
+
+// ContextDialID returns the DialID of the context, or zero
+func ContextDialID(ctx context.Context) int64 {
+	id, _ := ctx.Value(contextkey{}).(int64)
+	return id
+}

internal/engine/legacy/netx/dialid/dialid_test.go

@@ -0,0 +1,24 @@
+package dialid
+
+import (
+	"context"
+	"testing"
+)
+
+func TestGood(t *testing.T) {
+	ctx := context.Background()
+	id := ContextDialID(ctx)
+	if id != 0 {
+		t.Fatal("unexpected ID for empty context")
+	}
+	ctx = WithDialID(ctx)
+	id = ContextDialID(ctx)
+	if id != 1 {
+		t.Fatal("expected ID equal to 1")
+	}
+	ctx = WithDialID(ctx)
+	id = ContextDialID(ctx)
+	if id != 2 {
+		t.Fatal("expected ID equal to 2")
+	}
+}