chore: use external nim-ffi repository

[Ivansete-status]

Background

We started exposing LogosMessaging ffi some time ago. While the approach works, it implies having some duplicated code, when trying to apply the same ffi approach to other Nim projects.

For that reason, we've created the nim-ffi repository and now, we should start extracting ffi code from within logos-messaging-nim

Details

logos-messaging-nim should start using nim-ffi