rfc43: fix identity cache partition (#3566)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Original issue: https://github.com/smithy-lang/smithy-rs/issues/3427
RFC: https://github.com/smithy-lang/smithy-rs/pull/3544

## Description
<!--- Describe your changes in detail -->

Fixes the `SharedCredentialsProvider` and `SharedTokenProvider` to
re-use a consistent cache partition by default.
`SdkConfig` does not create an identity cache by default still, that
will need to be a follow on PR that gates it behind a new behavior
version. Ideally we do that with the stalled stream protection work in
https://github.com/smithy-lang/smithy-rs/pull/3527


## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Added new unit and integration tests.

## 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
smithy-rs codegen or runtime crates


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