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/cmd/jafar/flagx/stringarray.go

@@ -0,0 +1,46 @@
+// Package flagx contains extensions for the standard library
+// flag package. The code is adapted from github.com/m-lab/go and more
+// specifically from <https://git.io/JJ8UA>. This file is licensed under
+// version 2.0 of the Apache License <https://git.io/JJ8Ux>.
+package flagx
+
+import (
+	"fmt"
+	"strings"
+)
+
+// StringArray is a new flag type. It appends the flag parameter to an
+// `[]string` allowing the parameter to be specified multiple times or using ","
+// separated items. Unlike other Flag types, the default argument should almost
+// always be the empty array, because there is no way to remove an element, only
+// to add one.
+type StringArray []string
+
+// Get retrieves the value contained in the flag.
+func (sa StringArray) Get() interface{} {
+	return sa
+}
+
+// Set accepts a string parameter and appends it to the associated StringArray.
+// Set attempts to split the given string on commas "," and appends each element
+// to the StringArray.
+func (sa *StringArray) Set(s string) error {
+	f := strings.Split(s, ",")
+	*sa = append(*sa, f...)
+	return nil
+}
+
+// String reports the StringArray as a Go value.
+func (sa StringArray) String() string {
+	return fmt.Sprintf("%#v", []string(sa))
+}
+
+// Contains returns true when the given value equals one of the StringArray values.
+func (sa StringArray) Contains(value string) bool {
+	for _, v := range sa {
+		if v == value {
+			return true
+		}
+	}
+	return false
+}