chore: standardize on `cruet` for case conversion (#5663)
`cruet` supports both conversion to `camelCase`/`PascalCase`/etc and
pluralization, while `convert_case` only supports case conversion.
48ac1378 introduced `cruet` in psl. This comit makes
`mongodb-schema-connector` use `cruet` as well for consistency across
the codebase.