Avoid overflow in exponential backoff computation (#3229)

## Motivation and Context
aws-sdk-rust#960

## Description
Use checked_pow to avoid overflow in backoff computation

## Testing
- unit test

## 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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