Merge `feature/credential-features` branch to main (#4240)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Merging the feature branch containing work from the below two PRs:
* https://github.com/smithy-lang/smithy-rs/pull/4238
* https://github.com/smithy-lang/smithy-rs/pull/4224




## Checklist
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rather than leaving it unchecked -->
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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Co-authored-by: AWS SDK Rust Bot <97246200+aws-sdk-rust-ci@users.noreply.github.com>