Missing `PayloadChecksumKind` variant for `vpc-lattice-svcs`

[nmoutschen]

Describe the feature

The PayloadChecksumKind enum in aws-sigv4 only supports two variants at the moment: XAmzSha256 and NoHeader.

However, making SigV4 requests for VPC Lattice services requre to set the x-amz-content-sha256 header with a value UNSIGNED-PAYLOAD.

Documentation: https://docs.aws.amazon.com/vpc-lattice/latest/ug/sigv4-authenticated-requests.html

Use Case

This would enable support for SigV4 for VPC Lattice.

Proposed Solution

Add a new variant to PayloadChecksumKind that will inject an UNSIGNED-PAYLOAD value, and modify the rest of aws-sigv4 to use this information.

Other Information

No response

Acknowledgements

• [X] I may be able to implement this feature request
• [ ] This feature might incur a breaking change

A note for the community

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[nmoutschen]

After diving into the code, it looks like we could already do this, but not with the PayloadChecksumKind enum.

I'll let you fine folks decide if you see an opportunity to improve the SDK itself. Also happy to contribute with an example for this specific use-case. 😄

[Velfi]

Thanks for submitting this @nmoutschen.

To whomever picks up https://github.com/awslabs/smithy-rs/issues/1798, they should also take a look at implementing this too, since it touches the same code in the signer.

[rcoh]

As @nmoutschen notes you can do this by using SignableBody::UnsignedPayload when you invoke the SigV4 crate.