Improve the PT spec and how PTs interface with Tor

We want to make it easier for developers (and academics) to design and implement new pluggable transports and get them easily integrated with Tor so that we can have a well-functioning PT integration pipeline.

This is a large project that will consist of several things:

• We need to assess pain points with the current PT spec and desired features from a variety of PT developers.

• We might want to take a look at the PTv2 specification to see where features differ from our v1 and also which features seem to be liked or used by PT developers.

• We should think about how bridge distribution should factor into the PT specification. For example, some transports such as meek and snowflake handle "bridge" information differently than transports whose bridges are distributed through BridgeDB. This results in a different interaction with Tor, and we might consider modifying the spec with the snowflake/broker model in mind (ticket legacy/trac#29296 (moved)).

In general, we should improve our communication with the pluggable transports community to see what they need and figure out how to get more PTs integrated with Tor.