[C++ SDK] Consider converting high-level classes in iggy.hpp to rust-based enums

### Description

The current sdk does not create enums in rust and export them. Instead, it defines those in iggy.hpp and uses them to send strings across the ffi boundary. Consider adding enums to rust and exporting them to cpp side.

sub issue of #2100

### Component

Other

### Proposed solution

_No response_

### Alternatives considered

_No response_

Start with iggy.hpp and review parent issue #2100 to understand the high-level classes and current string-based FFI handling. Compare the Rust and C++ SDK sides, then define completion as representing the relevant enums in Rust and exporting them to C++ instead of sending strings across the boundary.

Written by the indexing model from the issue text.