Fix `sigv4-s3express` position in `auth_scheme_options` (#3498)

## Motivation and Context
Fixes an S3-related bug introduced when s3 Express was added

## Description
This PR fixes a bug where s3 Express auth flow was incorrectly used when
no auth schemes were available for an endpoint. This use case was
observed in test settings and manifested itself as a confusing behavior.

To address this issue, the fix ensures that an auth scheme for S3
express will be registered after that for sigv4 has been registered.

## Testing
Added an integration test
`s3_express_auth_flow_should_not_be_reached_with_no_auth_schemes`

## Checklist
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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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