fix(oonimkall): allow Android app to set proxy (#303)

Part of https://github.com/ooni/probe/issues/985

pkg/oonimkall/session.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"net/url"
 	"runtime"
 	"sync"
 
@@ -65,6 +66,19 @@ type SessionConfig struct {
 	// then the session will not emit any log message.
 	Logger Logger
 
+	// Proxy allows you to optionally force a specific proxy
+	// rather than using no proxy (the default).
+	//
+	// Use `psiphon:///` to force using Psiphon with the
+	// embedded configuration file. Not all builds have
+	// an embedded configuration file, but OONI builds have
+	// such a file, so they can use this functionality.
+	//
+	// Use `socks5://10.0.0.1:9050/` to connect to a SOCKS5
+	// proxy running on 10.0.0.1:9050. This could be, for
+	// example, a suitably configured `tor` instance.
+	Proxy string
+
 	// ProbeServicesURL allows you to optionally force the
 	// usage of an alternative probe service instance. This setting
 	// should only be used for implementing integration tests.
@@ -153,10 +167,22 @@ func newSessionWithContext(ctx context.Context, config *SessionConfig) (*Session
 			Type:    "https",
 		})
 	}
+
+	// TODO(bassosimone): write tests for this functionality.
+	var proxyURL *url.URL
+	if config.Proxy != "" {
+		var err error
+		proxyURL, err = url.Parse(config.Proxy)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	engineConfig := engine.SessionConfig{
 		AvailableProbeServices: availableps,
 		KVStore:                kvstore,
 		Logger:                 newLogger(config.Logger, config.Verbose),
+		ProxyURL:               proxyURL,
 		SoftwareName:           config.SoftwareName,
 		SoftwareVersion:        config.SoftwareVersion,
 		TempDir:                config.TempDir,