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/experiment/webconnectivity/internal/internal.go

@@ -0,0 +1,23 @@
+// Package internal contains internal code.
+package internal
+
+import "fmt"
+
+// StringPointerToString converts a string pointer to a string. When the
+// pointer is null, we return the "nil" string.
+func StringPointerToString(v *string) (out string) {
+	out = "nil"
+	if v != nil {
+		out = fmt.Sprintf("%+v", *v)
+	}
+	return
+}
+
+// BoolPointerToString is like StringPointerToString but for bool.
+func BoolPointerToString(v *bool) (out string) {
+	out = "nil"
+	if v != nil {
+		out = fmt.Sprintf("%+v", *v)
+	}
+	return
+}

internal/engine/experiment/webconnectivity/internal/internal_test.go

@@ -0,0 +1,31 @@
+package internal_test
+
+import (
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity/internal"
+)
+
+func TestStringPointerToString(t *testing.T) {
+	s := "ANTANI"
+	if internal.StringPointerToString(&s) != s {
+		t.Fatal("unexpected result")
+	}
+	if internal.StringPointerToString(nil) != "nil" {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestBoolPointerToString(t *testing.T) {
+	v := true
+	if internal.BoolPointerToString(&v) != "true" {
+		t.Fatal("unexpected result")
+	}
+	v = false
+	if internal.BoolPointerToString(&v) != "false" {
+		t.Fatal("unexpected result")
+	}
+	if internal.BoolPointerToString(nil) != "nil" {
+		t.Fatal("unexpected result")
+	}
+}