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/oonimkall/tasks/event.go

@@ -0,0 +1,57 @@
+package tasks
+
+type eventEmpty struct{}
+
+// EventFailure contains information on a failure.
+type EventFailure struct {
+	Failure string `json:"failure"`
+}
+
+// EventLog is an event containing a log message.
+type EventLog struct {
+	LogLevel string `json:"log_level"`
+	Message  string `json:"message"`
+}
+
+type eventMeasurementGeneric struct {
+	Failure string `json:"failure,omitempty"`
+	Idx     int64  `json:"idx"`
+	Input   string `json:"input"`
+	JSONStr string `json:"json_str,omitempty"`
+}
+
+type eventStatusEnd struct {
+	DownloadedKB float64 `json:"downloaded_kb"`
+	Failure      string  `json:"failure"`
+	UploadedKB   float64 `json:"uploaded_kb"`
+}
+
+type eventStatusGeoIPLookup struct {
+	ProbeASN         string `json:"probe_asn"`
+	ProbeCC          string `json:"probe_cc"`
+	ProbeIP          string `json:"probe_ip"`
+	ProbeNetworkName string `json:"probe_network_name"`
+}
+
+// EventStatusProgress reports progress information.
+type EventStatusProgress struct {
+	Message    string  `json:"message"`
+	Percentage float64 `json:"percentage"`
+}
+
+type eventStatusReportGeneric struct {
+	ReportID string `json:"report_id"`
+}
+
+type eventStatusResolverLookup struct {
+	ResolverASN         string `json:"resolver_asn"`
+	ResolverIP          string `json:"resolver_ip"`
+	ResolverNetworkName string `json:"resolver_network_name"`
+}
+
+// Event is an event emitted by a task. This structure extends the event
+// described by MK v0.10.9 FFI API (https://git.io/Jv4Rv).
+type Event struct {
+	Key   string      `json:"key"`
+	Value interface{} `json:"value"`
+}