feat: improve string literal typings

[paul-vd]

This will allow intellisense to understand available string literals while allowing custom string values, see https://twitter.com/Nemuksis/status/1693909021143532005

Notes

• The first commit adds the logic which appends the &{} to string literals (feat: improve string literal typings)
• The second commit updates all the .d.ts declarations (chore: update .d.ts typings)
• The third commit adds the changelog (chore: update changelog)

Fixes

https://github.com/aws/aws-sdk-js/pull/4268 (https://github.com/aws/aws-sdk-js/issues/4155)

Checklist

• [X] .d.ts file is updated
• [X] changelog is added, npm run add-change