Add placeholder types for S3 Express and enable control flow to be redirected for S3 Express use case

[ysaito1001]

Motivation and Context

This PR is the first in the series to support the S3 Express feature in the Rust SDK. The work will be done in the feature branch, and once it is code complete, the branch will be merged to main.

Description

This PR adds placeholder types for S3 Express and enables control flow to be redirected for S3 Express use case. For instance, if we run the following example code against a generated SDK from this PR:

let shared_config = aws_config::from_env().region(aws_sdk_s3::config::Region::new("us-east-1")).load().await;
let client = aws_sdk_s3::Client::new(&shared_config);
client.list_objects_v2().bucket("testbucket--use1-az4--x-s3";).send().await.unwrap();

it will end up

thread 's3_express' panicked at 'not yet implemented', /Users/awsaito/src/smithy-rs/aws/sdk/build/aws-sdk/sdk/s3/src/s3_express.rs:104:13

which points to

impl ProvideCredentials for DefaultS3ExpressIdentityProvider {
    fn provide_credentials<'a>(&'a self) -> aws_credential_types::provider::future::ProvideCredentials<'a>
    where
        Self: 'a,
    {
        todo!() <---
    }
}

Implementation decisions

• DefaultS3ExpressIdentityProvider has an accompanying identity cache. That identity cache cannot be configured by customers so it makes sense for the provider itself to internally own it. In that case, we do NOT want to use the identity cache stored in RuntimeComponents, since it interferes with the S3 Express's caching policy. To that end, I added an enum CacheLocation to SharedIdentityResolver (it already had the cache_partition field so it was kind of aware of caching).
  • Two reasons why CacheLocation is added to SharedIdentityResolver, but not to individual, concrete IdentityResolvers. One is SharedIdentityResolver was already cache aware, as mentioned above. The other is that it is more flexible that way; The cache location is not tied to a type of identity resolver, but we can select it when creating a SharedIdentityResolver.
  • I considered but did not add a field cacheable to Identity since I wanted to keep Identity as plain data, keeping the concept of "caching" somewhere outside.
• I've added a separate Config method, set_express_credentials_provider, to override credentials provider for S3 Express. There are other SDKs (e.g. Ruby) that follow this style and it makes it clear to the customers that this is the method to use when overriding the express credentials provider. The existing set_credentials_provider, given its input type, cannot tell whether a passed-in credentials provider is for a regular sigv4 or for S3 Express.

Testing

Only verified that control flow could be altered for an S3 Express use case, as shown above. Further testing will be added in subsequent PRs.

Checklist

I am planning to include in CHANGELOG.next.toml a user guide for S3 Express once the feature branch ysaito/s3express is ready to be merged to main.

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[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.

[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.

[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.

[github-actions]

A new generated diff is ready to view.

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

A new doc preview is ready to view.