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/tunnel/tunnel.go

@@ -124,6 +124,8 @@ func Start(ctx context.Context, config *Config) (Tunnel, DebugInfo, error) {
 		return fakeStart(ctx, config)
 	case "psiphon":
 		return psiphonStart(ctx, config)
+	case "torsf":
+		return torsfStart(ctx, config)
 	case "tor":
 		return torStart(ctx, config)
 	default: