ConnectAndWait connects to the socket passed via well-known env var, if present, and attempts to read from it until it receives an EOF, at which point cb is called.

SetupConn sets up a Unix socket listener, establishes a goroutine to handle connections and update the conn pointer, and returns the listener for the socket (which the caller is responsible for closing when it's no longer needed).

EnvKey represents the well-known environment variable used to pass the plugin being executed the socket name it should listen on to coordinate with the host CLI.