Fix `config::Builder::set_credentials_provider` to override what's previsouly set (#3278)

## Motivation and Context
Fixes https://github.com/awslabs/aws-sdk-rust/issues/973

## Description
Prior to the PR, if a customer explicitly passed a credentials provider
to a client's config `Builder::set_credentials_provider`, what's passed
did not override a credentials provider previously set ([actual use
case](https://github.com/awslabs/aws-sdk-rust/issues/973#issuecomment-1827224049)).

While in general, we recommend customers single-source a credentials
provider through
[aws_config::ConfigLoader::credentials_provider](https://docs.rs/aws-config/1.0.1/aws_config/struct.ConfigLoader.html#method.credentials_provider),
we should eliminate the said footgun in case they directly pass a
credentials provider to a client's config `Builder`.

The PR reverts test signature updates in
https://github.com/smithy-lang/smithy-rs/pull/3156

 (in hindsight, having
to update test signatures in that PR was a sign of regression).

## Testing
Added a Kotlin test to `CredentialProviderConfigTest.kt` to verify the
fix

## 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 AWS
SDK, generated SDK code, or SDK runtime crates

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Co-authored-by: John DiSanti <jdisanti@amazon.com>