Flexible Checksums V2 (#3967)

## Motivation and Context
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Flexible checksums V2 will be launching this week

## Description
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## Testing
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## Checklist
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- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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_By submitting this pull request, I confirm that you can use, modify,
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Co-authored-by: Zelda Hessler <zhessler@amazon.com>
Co-authored-by: Aaron Todd <aajtodd@users.noreply.github.com>