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/netx/resolver/dnsoverhttps.go

@@ -0,0 +1,77 @@
+package resolver
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io/ioutil"
+	"net/http"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/httpheader"
+)
+
+// DNSOverHTTPS is a DNS over HTTPS RoundTripper. Requests are submitted over
+// an HTTP/HTTPS channel provided by URL using the Do function.
+type DNSOverHTTPS struct {
+	Do           func(req *http.Request) (*http.Response, error)
+	URL          string
+	HostOverride string
+}
+
+// NewDNSOverHTTPS creates a new DNSOverHTTP instance from the
+// specified http.Client and URL, as a convenience.
+func NewDNSOverHTTPS(client *http.Client, URL string) DNSOverHTTPS {
+	return NewDNSOverHTTPSWithHostOverride(client, URL, "")
+}
+
+// NewDNSOverHTTPSWithHostOverride is like NewDNSOverHTTPS except that
+// it's creating a resolver where we use the specified host.
+func NewDNSOverHTTPSWithHostOverride(client *http.Client, URL, hostOverride string) DNSOverHTTPS {
+	return DNSOverHTTPS{Do: client.Do, URL: URL, HostOverride: hostOverride}
+}
+
+// RoundTrip implements RoundTripper.RoundTrip.
+func (t DNSOverHTTPS) RoundTrip(ctx context.Context, query []byte) ([]byte, error) {
+	ctx, cancel := context.WithTimeout(ctx, 45*time.Second)
+	defer cancel()
+	req, err := http.NewRequest("POST", t.URL, bytes.NewReader(query))
+	if err != nil {
+		return nil, err
+	}
+	req.Host = t.HostOverride
+	req.Header.Set("user-agent", httpheader.UserAgent())
+	req.Header.Set("content-type", "application/dns-message")
+	var resp *http.Response
+	resp, err = t.Do(req.WithContext(ctx))
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		// TODO(bassosimone): we should map the status code to a
+		// proper Error in the DNS context.
+		return nil, errors.New("doh: server returned error")
+	}
+	if resp.Header.Get("content-type") != "application/dns-message" {
+		return nil, errors.New("doh: invalid content-type")
+	}
+	return ioutil.ReadAll(resp.Body)
+}
+
+// RequiresPadding returns true for DoH according to RFC8467
+func (t DNSOverHTTPS) RequiresPadding() bool {
+	return true
+}
+
+// Network returns the transport network (e.g., doh, dot)
+func (t DNSOverHTTPS) Network() string {
+	return "doh"
+}
+
+// Address returns the upstream server address.
+func (t DNSOverHTTPS) Address() string {
+	return t.URL
+}
+
+var _ RoundTripper = DNSOverHTTPS{}