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/experiment/urlgetter/configurer.go

@@ -0,0 +1,102 @@
+package urlgetter
+
+import (
+	"crypto/tls"
+	"errors"
+	"net"
+	"net/url"
+	"regexp"
+	"strings"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
+)
+
+// The Configurer job is to construct a Configuration that can
+// later be used by the measurer to perform measurements.
+type Configurer struct {
+	Config   Config
+	Logger   model.Logger
+	ProxyURL *url.URL
+	Saver    *trace.Saver
+}
+
+// The Configuration is the configuration for running a measurement.
+type Configuration struct {
+	HTTPConfig netx.Config
+	DNSClient  netx.DNSClient
+}
+
+// CloseIdleConnections will close idle connections, if needed.
+func (c Configuration) CloseIdleConnections() {
+	c.DNSClient.CloseIdleConnections()
+}
+
+// NewConfiguration builds a new measurement configuration.
+func (c Configurer) NewConfiguration() (Configuration, error) {
+	// set up defaults
+	configuration := Configuration{
+		HTTPConfig: netx.Config{
+			BogonIsError:        c.Config.RejectDNSBogons,
+			CacheResolutions:    true,
+			CertPool:            c.Config.CertPool,
+			ContextByteCounting: true,
+			DialSaver:           c.Saver,
+			HTTP3Enabled:        c.Config.HTTP3Enabled,
+			HTTPSaver:           c.Saver,
+			Logger:              c.Logger,
+			ReadWriteSaver:      c.Saver,
+			ResolveSaver:        c.Saver,
+			TLSSaver:            c.Saver,
+		},
+	}
+	// fill DNS cache
+	if c.Config.DNSCache != "" {
+		entry := strings.Split(c.Config.DNSCache, " ")
+		if len(entry) < 2 {
+			return configuration, errors.New("invalid DNSCache string")
+		}
+		domainregex := regexp.MustCompile(`^([a-z0-9]+(-[a-z0-9]+)*\.)+[a-z]{2,}$`)
+		if !domainregex.MatchString(entry[0]) {
+			return configuration, errors.New("invalid domain in DNSCache")
+		}
+		var addresses []string
+		for i := 1; i < len(entry); i++ {
+			if net.ParseIP(entry[i]) == nil {
+				return configuration, errors.New("invalid IP in DNSCache")
+			}
+			addresses = append(addresses, entry[i])
+		}
+		configuration.HTTPConfig.DNSCache = map[string][]string{
+			entry[0]: addresses,
+		}
+	}
+	dnsclient, err := netx.NewDNSClientWithOverrides(
+		configuration.HTTPConfig, c.Config.ResolverURL,
+		c.Config.DNSHTTPHost, c.Config.DNSTLSServerName,
+		c.Config.DNSTLSVersion,
+	)
+	if err != nil {
+		return configuration, err
+	}
+	configuration.DNSClient = dnsclient
+	configuration.HTTPConfig.BaseResolver = dnsclient.Resolver
+	// configure TLS
+	configuration.HTTPConfig.TLSConfig = &tls.Config{
+		NextProtos: []string{"h2", "http/1.1"},
+	}
+	if c.Config.TLSServerName != "" {
+		configuration.HTTPConfig.TLSConfig.ServerName = c.Config.TLSServerName
+	}
+	err = netx.ConfigureTLSVersion(
+		configuration.HTTPConfig.TLSConfig, c.Config.TLSVersion,
+	)
+	if err != nil {
+		return configuration, err
+	}
+	configuration.HTTPConfig.NoTLSVerify = c.Config.NoTLSVerify
+	// configure proxy
+	configuration.HTTPConfig.ProxyURL = c.ProxyURL
+	return configuration, nil
+}