Use `Identity` instead of `Credentials` in signing code (#2913)

This PR replaces the access_key, secret_key, and session token fields of
signing params with the Orchestrator's `Identity` type.

## 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
- [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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