Add support for flexible checksums on Normal payloads

[dayaffe]

Tasks

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1. Add a wrapper for CRT provided hash functions
2. Add support for httpChecksum trait
3. Create a middleware that modifies request to include checksum hash in header
4. Create a middleware that validates checksum in response
5. Add tests for Normal payload

[sichanyoo]

Released with:
https://github.com/awslabs/aws-sdk-swift/releases/tag/0.35.0
https://github.com/smithy-lang/smithy-swift/releases/tag/0.40.0