Add a trait SignWith for signing HTTP requests

[ysaito1001]

Motivation and Context

#3388 (comment)

Description

This PR attempts to provide more ergonomic API for request signing. As documented in aws_sigv4, customers need to follow these steps after creating an Identity and a SigningParams:

// Convert the HTTP request into a signable request
let signable_request = SignableRequest::new(
    "GET",
    "https://some-endpoint.some-region.amazonaws.com",
    std::iter::empty(),
    SignableBody::Bytes(&[])
).expect("signable request");

let mut my_req = http::Request::new("...");
// Sign and then apply the signature to the request
let (signing_instructions, _signature) = sign(signable_request, &signing_params)?.into_parts();
signing_instructions.apply_to_request_http1x(&mut my_req);

This PR allows them to call SignWith::sign_with trait method with SigningPackage instead:

let signing_package = SigningPackage::builder()
    .signing_params(&signing_params)
    .build()
    .expect("required fields have been set");
my_req.sign_with(&signing_package);

Testing

Relied on existing tests in CI

Checklist

• I have updated CHANGELOG.next.toml if I made changes to the AWS SDK, generated SDK code, or SDK runtime crates

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[github-actions]

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• AWS SDK (ignoring whitespace)
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[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[jdisanti]

Why remove the comment?

[ysaito1001]

Oh removal was by mistake. Will restore it.

[jdisanti]

Needs its own error type.

[rcoh]

Overall I think that a better API for signing is truly needed and this is directionally good. However, I think we need to work backwards from customer use cases. For example: We should work backwards from a customer who wants to sign an API Gateway request (and write the example!) and handle this that way.

If this code is needed urgently for S3 Express, we could consider putting it in inlineable to iterate on it.

I'm not totally sure about the trait API either. On the surface, it seems like it allows for an implementation on foreign types but I don't think it does in practice due to orphan trait rules. At the very least we should compare with other API designs.

[ysaito1001]

As Russell commented, we need more iterations at a design level, and this PR will most likely be irrelevant when that design is settled. So closing this.

For S3 Express, there is a PR created to inline SigV4Signer::sign_http_request into S3ExpressSigner::sign_http_request.