getTopicId(topicName) {
    const region = this.serverless.getProvider('aws').getRegion()
    const stage = this.serverless.getProvider('aws').getStage()
    const serviceName = this.serverless.service.getServiceName()

    let topicId = `sls/${region}/${serviceName}/${stage}`

    if (topicName) {
      topicId += `/${topicName}`
    }

    return topicId
  }
Given a topic name, constructs the full topic identifier for the IoT endpoint.
The topic identifier is used to subscribe to the IoT endpoint and listen for incoming events.
The topic identifier is constructed as "sls/region/serviceName/stageName/functionName".
@param {string} topicName
@returns {string} topicId - The full topic identifier for the IoT endpoint.