The new `ipc` module inside `tor-ptmgr` implements the Pluggable
Transport Specification version 1 (`pt-spec.txt`,
https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/pt-spec.txt).
This enables module users to spawn pluggable transport binaries inside a
child process asynchronously, and receive structured information about
how to connect to the transports provided by said binaries.
Internally, this is structured as a pure set of serialisers and
deserialisers for the protocol in the specification (in the form of
environment variables, and the PT's stdout), a wrapper to run the PT
binary and enable accessing its output asynchronously, and a user-facing
wrapper that handles ensuring all the requested transports launched
properly.
The included `run-pt` example is an exceedingly minimal wrapper program
that was useful in testing. More tests can and should be added in a
further MR.
closes arti#394; part of arti#69
a118a639ed