add traceparent to unsignableHeaders

[Ankcorn]

Checklist

• [x] npm run test passes
• [ ] .d.ts file is updated
• [ ] changelog is added, npm run add-change
• [ ] run bundle exec rake docs:api and inspect doc/latest/index.html if documentation is changed
• [ ] run npm run integration if integration test is changed
• [ ] non-code related change (markdown/git settings etc)

Fixes https://github.com/aws/aws-sdk-js/issues/4472