feat: add github.com/tailscale/hujson extensions (#1294)

The hujson is a JSON package from tailscale allowing JSON files to
contain comments that is also more liberal with respect to commas.

We want to slowly upgrade all user-facing JSON-parsing code to use this
more relaxed standard for parsing JSONs, because that would make our
user's life significantly simpler in terms of writing and updating those
files.

In the short term, I would like the configuration file driving the usage
of beacons to use this relaxed parsing.

To this end, I am adding handy wrappers that combine using hujson with
using the standard library parser to have a single hujson aware parsing
function that behaves like the standard library's json.Unmarshal.

Part of ooni/probe#2531

internal/hujsonx/hujsonx.go

@@ -0,0 +1,18 @@
+// Package hujsonx contains github.com/tailscale/hujson extensions.
+package hujsonx
+
+import (
+	"encoding/json"
+
+	"github.com/tailscale/hujson"
+)
+
+// Unmarshal is like [json.Unmarshal] except that it first removes comments and
+// extra commas using the [hujson.Standardize] function.
+func Unmarshal(data []byte, v any) error {
+	data, err := hujson.Standardize(data)
+	if err != nil {
+		return err
+	}
+	return json.Unmarshal(data, v)
+}

internal/hujsonx/hujsonx_test.go

@@ -0,0 +1,33 @@
+package hujsonx
+
+import (
+	"errors"
+	"io"
+	"testing"
+)
+
+type user struct {
+	Name string
+	Age  int
+}
+
+func TestHuJSONXWorkingAsIntended(t *testing.T) {
+	t.Run("for invalid input", func(t *testing.T) {
+		input := []byte("{")
+		var v user
+		err := Unmarshal(input, &v)
+		if !errors.Is(err, io.ErrUnexpectedEOF) {
+			t.Fatal("unexpected error", err)
+		}
+	})
+
+	t.Run("for valid JSON we cannot map to a real struct", func(t *testing.T) {
+		input := []byte(`{"Name": {}, "Age": []}`)
+		var v user
+		err := Unmarshal(input, &v)
+		expected := "json: cannot unmarshal object into Go struct field user.Name of type string"
+		if err == nil || err.Error() != expected {
+			t.Fatal("unexpected error", err)
+		}
+	})
+}