feature/s3/manager needs an overhaul

[lucix-aws]

Issues pertaining to the s3 transfer manager are by far the most common type of feature request we get in the v2 SDK.

Our intent is to launch a "v2"-type refresh of the transfer manager API to address the existing customer asks.

Overview of goals here (not necessarily exhaustive):

• Implement "progress listener" functionality (Add support for S3 transfer manager callbacks #941)
• Refactor general IO to be more go-idiomatic (Support for io.Reader Interface in S3 Transfer Manager's Downloader #2247)
• Implement bulk copy (Support Multipart Copy in transfer manager #2095)
• Implement bulk delete (s3manager.BatchDelete missing from SDK V2 #1463)

Additionally there is now an internal specification that defines how SDKs should implement an S3 "transfer manager" which describes the APIs it must include. Some of the requested features are not yet part of that specification. In these cases we intend to work within the org to formalize a specification for these APIs.