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/probeservices/metadata.go

@@ -0,0 +1,52 @@
+package probeservices
+
+// Metadata contains metadata about a probe. This message is
+// included into a bunch of messages sent to orchestra.
+type Metadata struct {
+	AvailableBandwidth string   `json:"available_bandwidth,omitempty"`
+	DeviceToken        string   `json:"device_token,omitempty"`
+	Language           string   `json:"language,omitempty"`
+	NetworkType        string   `json:"network_type,omitempty"`
+	Platform           string   `json:"platform"`
+	ProbeASN           string   `json:"probe_asn"`
+	ProbeCC            string   `json:"probe_cc"`
+	ProbeFamily        string   `json:"probe_family,omitempty"`
+	ProbeTimezone      string   `json:"probe_timezone,omitempty"`
+	SoftwareName       string   `json:"software_name"`
+	SoftwareVersion    string   `json:"software_version"`
+	SupportedTests     []string `json:"supported_tests"`
+}
+
+// Valid returns true if metadata is valid, false otherwise. Metadata is
+// considered valid if all the mandatory fields are not empty. If a field
+// is marked `json:",omitempty"` in the structure definition, then it's
+// for sure mandatory. The "device_token" field is mandatory only if the
+// platform is "ios" or "android", because there's currently no device
+// token that we know of for desktop devices.
+func (m Metadata) Valid() bool {
+	if m.ProbeCC == "" {
+		return false
+	}
+	if m.ProbeASN == "" {
+		return false
+	}
+	if m.Platform == "" {
+		return false
+	}
+	if m.SoftwareName == "" {
+		return false
+	}
+	if m.SoftwareVersion == "" {
+		return false
+	}
+	if len(m.SupportedTests) < 1 {
+		return false
+	}
+	switch m.Platform {
+	case "ios", "android":
+		if m.DeviceToken == "" {
+			return false
+		}
+	}
+	return true
+}