Support KMS encryption context for S3 transfer commands

[robbie-demuth]

Description of changes:

Supports KMS encryption context for S3 transfer commands (s3 sync, s3 cp, and s3 mv)

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[robbie-demuth]

@kdaily and/or @kyleknap, mind giving this a review? First time contributor here, so feel free to redirect me if I'm not following the process correctly. I took a look at the contributing guide and didn't see anything about requesting reviews. Thanks!