BridgeDB should pass pluggable transport shared-secrets to clients

To correctly deploy pluggable transports like scramblesuit (that use shared-secrets), BridgeDB will need to get the shared-secrets out of the extra-info descriptor, using the arglist argument of the transport line as explained here: https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/180-pluggable-transport.txt#l454