Make profile keys case-insensitive (#3344)

## Motivation and Context
- https://github.com/aws/aws-sdk/issues/574
- https://github.com/awslabs/aws-sdk-rust/issues/1020

## Description
To match the CLI behavior and improve customer experience, make the keys
on the profile file case-insensitive.

## Testing
- New credentials integration test

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] 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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