add support for pre_start and post_start scripts

[prabirshrestha]

Added pre_start and post_start scripting support in stack-deploy.toml.

This will allow us to run custom scripts and can be useful when setting us networks for reverse proxy that is external.

Example for pre_start script for creating a network for traefik.

#!/bin/sh
NETWORK_NAME="reverse_proxy"

if ! docker network ls --filter name=^${NETWORK_NAME}$ --format "{{.Name}}" | grep -wq ${NETWORK_NAME}; then
  echo "Network '${NETWORK_NAME}' does not exist. Creating it now..."
  docker network create ${NETWORK_NAME}
else
  echo "Network '${NETWORK_NAME}' already exists."
fi

I also pass the secret_env so this could be used to replace values in config files that is outside of compose.yml.