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/libooniffi/testdata/webconnectivity.json

@@ -0,0 +1,17 @@
+{
+  "assets_dir": ".",
+  "inputs": [
+    "https://www.example.com",
+    "https://www.example.org"
+  ],
+  "name": "WebConnectivity",
+  "log_level": "INFO",
+  "options": {
+    "no_collector": true,
+    "software_name": "ooniffi",
+    "software_version": "0.1.0-dev"
+  },
+  "state_dir": ".",
+  "temp_dir": ".",
+  "version": 1
+}