Add support for `.service` file

Start with manage_apps.py and deploy.sh, tracing the existing APP and METHOD environment-variable flow. Review the proposed SERVICEFILEPATH handling and connection.serviceExists checks, then follow how deploy.sh invokes manage_apps.py. Done means a .service file is recognized during deploy previews and the described service checks and creation flow works for applications needing services such as Redis or Postgres.

For applications which require services like Redis or Postgres, a standard file (probably .service) would be useful to recognize and automatically spin up those services for the app. This is especially useful as part of the deploy preview, which would be frustrating to use for now for apps which depend on those resources.

We can accomplish this with logic along the lines of:

SERVICE_FILEPATH = os.environ.get("SERVICEFILEPATH")
METHOD = os.environ.get("METHOD")

if SERVICE_FILEPATH:
    with open(SERVICE_FILEPATH, 'r') as svc_file:
        service_configs = json.load(svc_file)
        if not isinstance(service_configs, list):
            service_configs = [service_configs]

if METHOD == "CHECKSVC":
    for service in service_configs:
        push = "false" if connection.serviceExists(APP, service['type']) else "true"
        if push == "true":
            print(push)

in manage_apps.py and logic along the lines of:

# Check if service needs to be created
if [[ -f "$APPS_DIR/$APP/.service" ]]; then
  service_file="$APPS_DIR/$APP/.service"
  create_svc=true
fi

if [[ "$create_svc" == "true" ]]; then
  APP=$with_suffix METHOD="CREATESVC" SERVICEFILEPATH=$service_file python ./manage_apps.py
fi

in deploy.sh.