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/httphostheader/httphostheader.go

@@ -0,0 +1,94 @@
+// Package httphostheader contains the HTTP host header network experiment.
+//
+// This experiment has not been specified yet. It is nonetheless available for testing
+// and as a building block that other experiments could reuse.
+package httphostheader
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/urlgetter"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+const (
+	testName    = "http_host_header"
+	testVersion = "0.3.0"
+)
+
+// Config contains the experiment config.
+type Config struct {
+	// TestHelperURL is the address of the test helper.
+	TestHelperURL string
+}
+
+// TestKeys contains httphost test keys.
+type TestKeys struct {
+	urlgetter.TestKeys
+	THAddress string `json:"th_address"`
+}
+
+// Measurer performs the measurement.
+type Measurer struct {
+	config Config
+}
+
+// ExperimentName implements ExperimentMeasurer.ExperiExperimentName.
+func (m *Measurer) ExperimentName() string {
+	return testName
+}
+
+// ExperimentVersion implements ExperimentMeasurer.ExperimentVersion.
+func (m *Measurer) ExperimentVersion() string {
+	return testVersion
+}
+
+// Run implements ExperimentMeasurer.Run.
+func (m *Measurer) Run(
+	ctx context.Context,
+	sess model.ExperimentSession,
+	measurement *model.Measurement,
+	callbacks model.ExperimentCallbacks,
+) error {
+	if measurement.Input == "" {
+		return errors.New("experiment requires input")
+	}
+	if m.config.TestHelperURL == "" {
+		m.config.TestHelperURL = "http://www.example.org"
+	}
+	urlgetter.RegisterExtensions(measurement)
+	g := urlgetter.Getter{
+		Begin: measurement.MeasurementStartTimeSaved,
+		Config: urlgetter.Config{
+			HTTPHost: string(measurement.Input),
+		},
+		Session: sess,
+		Target:  fmt.Sprintf(m.config.TestHelperURL),
+	}
+	tk, _ := g.Get(ctx)
+	measurement.TestKeys = &TestKeys{
+		TestKeys:  tk,
+		THAddress: m.config.TestHelperURL,
+	}
+	return nil
+}
+
+// NewExperimentMeasurer creates a new ExperimentMeasurer.
+func NewExperimentMeasurer(config Config) model.ExperimentMeasurer {
+	return &Measurer{config: config}
+}
+
+// SummaryKeys contains summary keys for this experiment.
+//
+// Note that this structure is part of the ABI contract with probe-cli
+// therefore we should be careful when changing it.
+type SummaryKeys struct {
+	IsAnomaly bool `json:"-"`
+}
+
+// GetSummaryKeys implements model.ExperimentMeasurer.GetSummaryKeys.
+func (m Measurer) GetSummaryKeys(measurement *model.Measurement) (interface{}, error) {
+	return SummaryKeys{IsAnomaly: false}, nil
+}