Add clippy.toml with forbidden methods & fix SystemTime usages (#2882)

## Motivation and Context
- #2087 

## Description
- remove lingering usages of SystemTime::now()
- ensure they don't return by adding a clippy.toml

## Testing
- CI
- Ran the webassembly example

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] 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: John DiSanti <jdisanti@amazon.com>