Create a submodule to handle commercetools messages (events) using cloud providers

Problem/Motivation

Now we have implemented handling of the commercetools messages using GraphQL API. However, some types of messages are not provided in the GraphQL API and are present only in the cloud-based message queue APIs (Amazon SQS, SNS, Google Pub/Sub, etc).

More details: https://github.com/commercetools/commercetools-sdk-php-v2/issues/302

Proposed resolution

Would be great to implement a submodule (to not introduce new dependencies in the main module) that listens for messages from the cloud-based message queue APIs.