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/model/measurement.go

@@ -0,0 +1,126 @@
+package model
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// MeasurementTarget is the target of a OONI measurement.
+type MeasurementTarget string
+
+// MarshalJSON serializes the MeasurementTarget.
+func (t MeasurementTarget) MarshalJSON() ([]byte, error) {
+	if t == "" {
+		return json.Marshal(nil)
+	}
+	return json.Marshal(string(t))
+}
+
+// Measurement is a OONI measurement.
+//
+// This structure is compatible with the definition of the base data format in
+// https://github.com/ooni/spec/blob/master/data-formats/df-000-base.md.
+type Measurement struct {
+	// Annotations contains results annotations
+	Annotations map[string]string `json:"annotations,omitempty"`
+
+	// DataFormatVersion is the version of the data format
+	DataFormatVersion string `json:"data_format_version"`
+
+	// Extensions contains information about the extensions included
+	// into the test_keys of this measurement.
+	Extensions map[string]int64 `json:"extensions,omitempty"`
+
+	// ID is the locally generated measurement ID
+	ID string `json:"id,omitempty"`
+
+	// Input is the measurement input
+	Input MeasurementTarget `json:"input"`
+
+	// InputHashes contains input hashes
+	InputHashes []string `json:"input_hashes,omitempty"`
+
+	// MeasurementStartTime is the time when the measurement started
+	MeasurementStartTime string `json:"measurement_start_time"`
+
+	// MeasurementStartTimeSaved is the moment in time when we
+	// started the measurement. This is not included into the JSON
+	// and is only used within probe-engine as a "zero" time.
+	MeasurementStartTimeSaved time.Time `json:"-"`
+
+	// Options contains command line options
+	Options []string `json:"options,omitempty"`
+
+	// ProbeASN contains the probe autonomous system number
+	ProbeASN string `json:"probe_asn"`
+
+	// ProbeCC contains the probe country code
+	ProbeCC string `json:"probe_cc"`
+
+	// ProbeCity contains the probe city
+	ProbeCity string `json:"probe_city,omitempty"`
+
+	// ProbeIP contains the probe IP
+	ProbeIP string `json:"probe_ip,omitempty"`
+
+	// ProbeNetworkName contains the probe network name
+	ProbeNetworkName string `json:"probe_network_name"`
+
+	// ReportID contains the report ID
+	ReportID string `json:"report_id"`
+
+	// ResolverASN is the ASN of the resolver
+	ResolverASN string `json:"resolver_asn"`
+
+	// ResolverIP is the resolver IP
+	ResolverIP string `json:"resolver_ip"`
+
+	// ResolverNetworkName is the network name of the resolver.
+	ResolverNetworkName string `json:"resolver_network_name"`
+
+	// SoftwareName contains the software name
+	SoftwareName string `json:"software_name"`
+
+	// SoftwareVersion contains the software version
+	SoftwareVersion string `json:"software_version"`
+
+	// TestHelpers contains the test helpers. It seems this structure is more
+	// complex than we would like. In particular, using a map from string to
+	// string does not fit into the web_connectivity use case. Hence, for now
+	// we're going to represent this using interface{}. In going forward we
+	// may probably want to have more uniform test helpers.
+	TestHelpers map[string]interface{} `json:"test_helpers,omitempty"`
+
+	// TestKeys contains the real test result. This field is opaque because
+	// each experiment will insert here a different structure.
+	TestKeys interface{} `json:"test_keys"`
+
+	// TestName contains the test name
+	TestName string `json:"test_name"`
+
+	// MeasurementRuntime contains the measurement runtime. The JSON name
+	// is test_runtime because this is the name expected by the OONI backend
+	// even though that name is clearly a misleading one.
+	MeasurementRuntime float64 `json:"test_runtime"`
+
+	// TestStartTime contains the test start time
+	TestStartTime string `json:"test_start_time"`
+
+	// TestVersion contains the test version
+	TestVersion string `json:"test_version"`
+}
+
+// AddAnnotations adds the annotations from input to m.Annotations.
+func (m *Measurement) AddAnnotations(input map[string]string) {
+	for key, value := range input {
+		m.AddAnnotation(key, value)
+	}
+}
+
+// AddAnnotation adds a single annotations to m.Annotations.
+func (m *Measurement) AddAnnotation(key, value string) {
+	if m.Annotations == nil {
+		m.Annotations = make(map[string]string)
+	}
+	m.Annotations[key] = value
+}