Implement S3 import worker

We need to port tasks.import_s3 as a Bucket import worker. Command will be bucket_import.

The ProcessTask should inherit from BaseWorker.

Do not port the methods you don't need (download_files,...).

Parse the following parameters from self.process_information:

• folder_type
• page_type from element_type
• prefix
• element

Parse the following parameters from worker configuration (no need to setup user configuration as these will not be exposed):

• bucket
• iiif_base_url
• bucket_prefix

INGEST_S3_* variables have to be available at runtime, raise if that's not the case.

Refs https://redmine.teklia.com/issues/7339