geolocate: add cloudflare-based IP lookup (#676)

Cloudflare hosted services provide a certain service of `/cdn-cgi/trace` with their base url (for example, `www.cloudflare.com` or `www.nginx.com`), which can be used to obtain `ip` in the probe's `geolocate` feature.

The same feature was added in this pr, hence, increasing the number of `baseURL`s in `geolocate`.

Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/engine/geolocate/cloudflare_test.go

@@ -0,0 +1,26 @@
+package geolocate
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/httpheader"
+)
+
+func TestIPLookupWorksUsingcloudlflare(t *testing.T) {
+	ip, err := cloudflareIPLookup(
+		context.Background(),
+		http.DefaultClient,
+		log.Log,
+		httpheader.UserAgent(),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if net.ParseIP(ip) == nil {
+		t.Fatalf("not an IP address: '%s'", ip)
+	}
+}