feat(miniooni): implement torsf tunnel (#921)

This diff adds to miniooni support for using the torsf tunnel. Such a
tunnel consists of a snowflake pluggable transport in front of a custom
instance of tor and requires tor to be installed.

The usage is like:

```
./miniooni --tunnel=torsf [...]
```

The default snowflake rendezvous method is "domain_fronting". You can
select the AMP cache instead using "amp":

```
./miniooni --snowflake-rendezvous=amp --tunnel=torsf [...]
```

Part of https://github.com/ooni/probe/issues/1955

internal/cmd/miniooni/session.go

@@ -36,14 +36,15 @@ func newSessionOrPanic(ctx context.Context, currentOptions *Options,
 	runtimex.PanicOnError(err, "cannot create tunnelDir")
 
 	config := engine.SessionConfig{
-		KVStore:         kvstore,
-		Logger:          logger,
-		ProxyURL:        proxyURL,
-		SoftwareName:    softwareName,
-		SoftwareVersion: softwareVersion,
-		TorArgs:         currentOptions.TorArgs,
-		TorBinary:       currentOptions.TorBinary,
-		TunnelDir:       tunnelDir,
+		KVStore:             kvstore,
+		Logger:              logger,
+		ProxyURL:            proxyURL,
+		SnowflakeRendezvous: currentOptions.SnowflakeRendezvous,
+		SoftwareName:        softwareName,
+		SoftwareVersion:     softwareVersion,
+		TorArgs:             currentOptions.TorArgs,
+		TorBinary:           currentOptions.TorBinary,
+		TunnelDir:           tunnelDir,
 	}
 	if currentOptions.ProbeServicesURL != "" {
 		config.AvailableProbeServices = []model.OOAPIService{{