[DRAFT] Experimental minimal S3 plugin

[jeqo]

Depends on #169

Implements an S3 RSM directly using the S3 client.

The purpose of this plugin implementation is to

• gain experience with the S3 client
• benchmark the features included on new-implementation branch (e.g. chunking, encryption, compression) against this to evaluate performance.

Not meant to be merged at the moment, as main work is going on new-implementation branch.

[jeqo]

will be partially included into S3 benchmarks, e.g. #273