[Analytics] Support bulk write via S3

I would like to propose a new write option in Cassandra Analytics to bulk write SSTables via S3, in addition to the previously-implemented "direct upload to all sidecars" (now known as the "Direct" transport).
The new write option, now being implemented, is the "S3_COMPAT" transport, which allows the job to upload the generated SSTables to an S3-compatible storage system, and then inform the Cassandra Sidecar that those files are available for download & commit.

Additionally, a plug-in system was added to allow communications between custom transport hooks and the job, so the custom hook can provide updated credentials and out-of-band status updates on S3-related issues.