spec: Add to pt-spec.txt control messages going back to main process (tor)

As part of legacy/trac#28180 (moved), we want to have a PT to report back events, mainly connection to the bridge events for now, to the parent process which is tor in our case.

This ticket is for adding those to pt-spec.txt.

We'll concentrate on connection messages reporting the status of a connection to a Bridge at first to narrow down a bit this work.