function getSnsSpec(shim, original, name, args) {
  const [command] = args
  return new MessageSpec({
    promise: true,
    callback: shim.LAST,
    destinationName: getDestinationName(command.input),
    destinationType: shim.TOPIC,
    opaque: true
  })
}
Returns the spec for PublishCommand

@param {Shim} shim
@param {original} original original middleware function
@param name
@param {Array} args to the middleware function
@returns {Object}