fix: remove unneeded Sync bounds

[dignifiedquire]

These Sync bounds make it hard to integrate with other tooling, and don't seem to be actually needed.

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[dignifiedquire]

Note: looks like the non core s3s crates still require it, will look into if I can fix this

[dignifiedquire]

probably not easily mergeable given the Sync requirements from the aws sdk/hyper-body