Create CloudStorage classes to abstract file upload/download

[medvedev1088]

Right now we support S3 and GCS as cloud storage providers and file upload/download is done directly in export DAG. Instead 2 classes S3CloudStorage and GCSCloudStorage can be created with common interface to abstract upload/download logic.

As an example:

class S3CloudStorage:
     def download_file(...)
     def upload_file(...)

upload_to_gcs and download_from_gcs can be moved from export DAG to GCSCloudStorage.