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.

----

_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>

Author: 3 people

.changelog/flexible-checksums-client.md

@@ -0,0 +1,10 @@
+---
+applies_to: ["client"]
+authors: ["landonxjames"]
+references: ["smithy-rs#3967"]
+breaking: false
+new_feature: true
+bug_fix: true
+---
+
+Updates client generation to conform with Smithy's updates to the [httpChecksum trait](https://smithy.io/2.0/aws/aws-core.html#aws-protocols-httpchecksum-trait).

.changelog/flexible-checksums-s3.md

@@ -0,0 +1,12 @@
+---
+applies_to: ["client", "aws-sdk-rust"]
+authors: ["landonxjames"]
+references: ["smithy-rs#3845"]
+breaking: false
+new_feature: true
+bug_fix: true
+---
+
+S3 client behavior is updated to always calculate a checksum by default for operations that support it (such as PutObject or UploadPart), or require it (such as DeleteObjects). The default checksum algorithm is CRC32. Checksum behavior can be configured using `when_supported` and `when_required` options - in shared config using request_checksum_calculation, or as env variable using AWS_REQUEST_CHECKSUM_CALCULATION.
+
+The S3 client attempts to validate response checksums for all S3 API operations that support checksums. However, if the SDK has not implemented the specified checksum algorithm then this validation is skipped. Checksum validation behavior can be configured using `when_supported` and `when_required` options - in shared config using response_checksum_validation, or as env variable using AWS_RESPONSE_CHECKSUM_VALIDATION.