Write server-side pluggable transport options to extra-info descriptor

To correctly deploy pluggable transports that require pre-shared-secrets, like scramblesuit, we need to be able to pass the shared-secret of each bridge to BridgeDB.

This is not currently implemented, and we need to implement the ARGS option of the SMETHOD line as detailed in 180-pluggable-transport.txt.

This is a ticket for the Tor side of this task. That is, parsing the SMETHOD line and writing stuff to the extra-info descriptors.