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/saver.go

@@ -0,0 +1,69 @@
+package engine
+
+import (
+	"errors"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// Saver saves a measurement on some persistent storage.
+type Saver interface {
+	SaveMeasurement(m *model.Measurement) error
+}
+
+// SaverConfig is the configuration for creating a new Saver.
+type SaverConfig struct {
+	// Enabled is true if saving is enabled.
+	Enabled bool
+
+	// Experiment is the experiment we're currently running.
+	Experiment SaverExperiment
+
+	// FilePath is the filepath where to append the measurement as a
+	// serialized JSON followed by a newline character.
+	FilePath string
+
+	// Logger is the logger used by the saver.
+	Logger model.Logger
+}
+
+// SaverExperiment is an experiment according to the Saver.
+type SaverExperiment interface {
+	SaveMeasurement(m *model.Measurement, filepath string) error
+}
+
+// NewSaver creates a new instance of Saver.
+func NewSaver(config SaverConfig) (Saver, error) {
+	if !config.Enabled {
+		return fakeSaver{}, nil
+	}
+	if config.FilePath == "" {
+		return nil, errors.New("saver: passed an empty filepath")
+	}
+	return realSaver{
+		Experiment: config.Experiment,
+		FilePath:   config.FilePath,
+		Logger:     config.Logger,
+	}, nil
+}
+
+type fakeSaver struct{}
+
+func (fs fakeSaver) SaveMeasurement(m *model.Measurement) error {
+	return nil
+}
+
+var _ Saver = fakeSaver{}
+
+type realSaver struct {
+	Experiment SaverExperiment
+	FilePath   string
+	Logger     model.Logger
+}
+
+func (rs realSaver) SaveMeasurement(m *model.Measurement) error {
+	rs.Logger.Info("saving measurement to disk")
+	return rs.Experiment.SaveMeasurement(m, rs.FilePath)
+}
+
+var _ Saver = realSaver{}