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/geolocate/ipinfo.go

@@ -0,0 +1,33 @@
+package geolocate
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/httpheader"
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/httpx"
+)
+
+type ipInfoResponse struct {
+	IP string `json:"ip"`
+}
+
+func ipInfoIPLookup(
+	ctx context.Context,
+	httpClient *http.Client,
+	logger Logger,
+	userAgent string,
+) (string, error) {
+	var v ipInfoResponse
+	err := (httpx.Client{
+		Accept:     "application/json",
+		BaseURL:    "https://ipinfo.io",
+		HTTPClient: httpClient,
+		Logger:     logger,
+		UserAgent:  httpheader.CLIUserAgent(), // we must be a CLI client
+	}).GetJSON(ctx, "/", &v)
+	if err != nil {
+		return DefaultProbeIP, err
+	}
+	return v.IP, nil
+}