S3 content files downloader and injection in swh

This includes:

• s3 tasks producer
• s3 downloader and injection in swh worker

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Migrated from T319 (view on Phabricator)

assigned to @ardumont

added Antelink loader priority:High labels

changed title from S3 content files downloader to S3 content files downloader code

closed

changed title from S3 content files downloader code to S3 content files downloader and injection in swh

changed the description

marked this issue as related to #321 (closed)

added state:duplicate label

removed state:duplicate label