Thread through `BehaviorVersion` to S3 Express identity provider (#3478)

## Motivation and Context
A follow-up task that resolves one of the `TODO(Post S3Express release)`

## Description
This PR ensures that `BehaviorVersion` is threaded through from an outer
S3 client to the inner S3 client when `DefaultS3ExpressIdentityProvider`
is constructed.

I considered the alternative where `BehaviorVersion` would be stored in
`ConfigBag`using another flavor like `StoreOnce` and obtained from the
bag within
`DefaultS3ExpressIdentityProvider::express_session_credentials`. But
ensuring `StoreOnce` per `ConfigBag` was not enough for
`BehaviorVersion` since there could be nested `ConfigBag`s where each
`ConfigBag` was separate. `BehaviorVersion` needs to be unique across
those different `ConfigBag`s and the fact that we store
`BehaviorVersion` outside `RuntimeComponents` or `ConfigBag` is a
reasonable choice from that perspective (the ease of guaranteeing the
uniqueness of `BehaviorVersion`).

## Testing
Relied on existing tests in CI.

## 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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