[fix] Sleep not passed to ECS provider (#998)

* [fix] Sleep not passed to ECS provider

When constructing an ECS credential provider, sleep_impl was not properly passed along from the provider config. This adds a test that asserts that retries will be made, and fixes the bug.

* Fix clippy lint

* Update changelog

Co-authored-by: Zelda Hessler <zhessler@amazon.com>