Support Featurevisor v2.0

Hi 👋 

Recently, Featurevisor v2.0 was released and you can find more info here:

- Blog post: https://featurevisor.com/blog/v2-release/
- Migration guide from v1 to v2 (for users): https://featurevisor.com/docs/migrations/v2/
- Latest JavaScript SDK v2.0 guide: https://featurevisor.com/docs/sdks/javascript/

This is a feature request for Swift SDK to support v2.0.

Thanks ❤️ 