Design a version 2 Pluggable Transport spec.

The current PT spec works, but has a few design flaws that require breaking compatibility to fix. While doing this process further thought should (maybe?) be given to making it even easier for other applications to adopt.

In addition to the existing issues that will be re-parented to this ticket the new spec should:

• Maybe not prefix env vars with TOR_, since "Pluggable Transports aren't just a Tor thing". No strong opinion on this. If that's all that's kept people from using the spec, it's kind of silly.
• Your idea here.