Fix EcsCredentialsProvider to respect query params (#3977)

## Motivation and Context
https://github.com/awslabs/aws-sdk-rust/issues/1248, and implemented the
fix as prescribed.

## Testing
Added a request matching unit test to the `ecs` module to ensure that
query params are included in credential's HTTP request.

## Checklist
- [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.

----

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