Zod v4 support (#598)

* Adds `zod` v4.0.0 support while maintaining partial backwards compatibility with `v3` per the Zod guide: https://zod.dev/library-authors.
* If you need to still use zod v3, we suggest continuing to use restate-sdk-zod 1.8.3

Co-authored-by: Nathan Standiford <[email protected]>

Author: slinkydeveloper