Add support to disable S3 Express session auth

[ysaito1001]

Motivation and Context

Adds the ability to disable S3 Express session auth (causing it to use a regular sigv4 session auth instead).

Description

S3 Express One Zone is an opt out feature, and there are three ways to disable it (with the order of precedence as listed):

• through the disable_s3_express_session_auth method on an S3 client
• through an environment variable AWS_S3_DISABLE_EXPRESS_SESSION_AUTH
• through a profile file with a key s3_disable_express_session_auth (won't be supported until Support service specific configs appearing in the general section of a profile file awslabs/aws-sdk-rust#1073 is addressed)

If one of the places is set to true/false, then the subsequent places with lower precedence will not be considered.

Something to be aware of when setting the disable option through the environment variable. The environment variable is only checked during a client construction, meaning that if a customer sets it after the client has been created the SDK will not take the environment variable value into account, i.e. the following snippet will NOT disable the S3 Express session auth:

let config = aws_config::load_from_env().await;
let client = aws_sdk_s3::Client::new(&config);

// Set the env variable to true after an S3 client has been created
std::env::set_var("AWS_S3_DISABLE_EXPRESS_SESSION_AUTH", "true");

let _ = client
    .list_objects_v2()
    .bucket("s3express-test-bucket--usw2-az1--x-s3")
    .send()
    .await;

Testing

Added unit tests for S3ExpressRuntimePlugin and integration tests for verifying disabling S3 Express session auth.

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[rcoh]

Looks good—consider tweaking the new client plugins API.

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

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[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• 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)
• 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.