spec: Add to control-spec.txt some pluggable transport events
Because of #28181 (moved) that will make possible for a PT to send back connection messages to Tor, we would like to send those to the control port to for instance Tor Browser can pick them up and inform the user.
For now, we only have the event TRANSPORT_LAUNCHED which indicate how/when the PT was launched.
As #28180 (moved) stated, most of the messages for now will be about the PT connection to the bridge such as "initiated", "unable to connect", "disconnected", etc...
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