Ability to override the IMDS client in DefaultCredentialsChain

[kevinpark1217]

Motivation and Context

This is an alternative method for me to implement necessary retry and timeout for the IMDS client, rather than through environment variables and profile as attempted in #1821 and outlined in aws-sdk-rust#625.

This should give me enough flexibility to navigate around some limitations and be able to implement timeout and retry attempts in code.

Description

Simple additional helper method to set the IMDS client with custom retry and timeout settings.

Testing

let attempts = 10;
let timeout = Duration::from_secs(60);

let imds_client = imds::Client::builder()
    .max_attempts(attempts)
    .connect_timeout(timeout)
    .read_timeout(timeout)
    .build()
    .await?;

let chain = DefaultCredentialsChain::builder()
    .region(region)
    .imds_client(imds_client);

Checklist

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

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

[jdisanti]

This seems reasonable to me.

[jdisanti]

Thank you for the contribution!