Add `NoCredentialsCache` that offers no caching ability (#2720)

## Motivation and Context
Related to https://github.com/awslabs/aws-sdk-rust/issues/809

## Description
It has been discovered that when `AssumeRoleProvider` is used, the Rust
SDK emits `credentials cache miss occurred` twice per request. The
reason why that log is shown twice is illustrated in the following
diagram:

![Screenshot 2023-05-19 at 4 10 20
PM](https://github.com/awslabs/smithy-rs/assets/15333866/c6cce018-c821-4b46-8d47-b414af7b4d1e

)

One of the cache miss messages is due to the fact `AssumeRoleProvider`
internally uses an STS client, which, in turn, is wrapped by a
`LazyCredentialsCache` by default. However, that use of
`LazyCredentialsCache` is pointless because caching is already in effect
with the outermost `LazyCredentialsCache`.

This PR adds a new kind of `CredentialsCache`, `NoCredentialsCache`. As
its name suggests, it simplify delegates `provide_cached_credentials` to
the underlying provider's `provide_credentials` with no caching
functionality. We then update `SsoCredentialsProvider`,
`AssumeRoleProvider`, and `WebIdentityTokenCredentialsProvider` to use
`NoCredentialsCache` for their STS clients so the logs won't show
`credentials cache miss occurred` twice per request.

## Testing
- Added unit tests for `NoCredentialsCache`
- Updated unit test for `AssumeRoleProvider` to verify
`NoCredentialsCache` is used by default

## Checklist
- [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: Yuki Saito <awsaito@amazon.com>