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/internal/httpx/fetch.go

@@ -0,0 +1,31 @@
+package httpx
+
+import (
+	"context"
+	"crypto/sha256"
+	"fmt"
+)
+
+// FetchResource fetches the specified resource and returns it.
+func (c Client) FetchResource(ctx context.Context, URLPath string) ([]byte, error) {
+	request, err := c.NewRequest(ctx, "GET", URLPath, nil, nil)
+	if err != nil {
+		return nil, err
+	}
+	return c.Do(request)
+}
+
+// FetchResourceAndVerify fetches and verifies a specific resource.
+func (c Client) FetchResourceAndVerify(ctx context.Context, URL, SHA256Sum string) ([]byte, error) {
+	c.Logger.Debugf("httpx: expected SHA256: %s", SHA256Sum)
+	data, err := c.FetchResource(ctx, URL)
+	if err != nil {
+		return nil, err
+	}
+	s := fmt.Sprintf("%x", sha256.Sum256(data))
+	c.Logger.Debugf("httpx: real SHA256: %s", s)
+	if SHA256Sum != s {
+		return nil, fmt.Errorf("httpx: SHA256 mismatch: got %s and expected %s", s, SHA256Sum)
+	}
+	return data, nil
+}