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/checkreportid.go

@@ -0,0 +1,29 @@
+package probeservices
+
+import (
+	"context"
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/httpx"
+)
+
+type checkReportIDResponse struct {
+	Found bool `json:"found"`
+}
+
+// CheckReportID checks whether the given ReportID exists.
+func (c Client) CheckReportID(ctx context.Context, reportID string) (bool, error) {
+	query := url.Values{}
+	query.Add("report_id", reportID)
+	var response checkReportIDResponse
+	err := (httpx.Client{
+		BaseURL:    c.BaseURL,
+		HTTPClient: c.HTTPClient,
+		Logger:     c.Logger,
+		UserAgent:  c.UserAgent,
+	}).GetJSONWithQuery(ctx, "/api/_/check_report_id", query, &response)
+	if err != nil {
+		return false, err
+	}
+	return response.Found, nil
+}