allow to provide postcss plugin options as a string (#35173)
fixes https://github.com/vercel/next.js/issues/35117
@balazsorban44 this needs a test case
## Bug
- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
Co-authored-by: Balázs Orbán <18369201+balazsorban44@users.noreply.github.com>
