Add Option<String> support for shared struct fields

[Choochmeque]

Adds support for Option fields in shared structs.

Example

// rust
#[swift_bridge::bridge]
mod ffi {
    #[swift_bridge(swift_repr = "struct")]
    struct UserProfile {
        name: String,
        bio: Option<String>,  // Now supported!
    }

    extern "Rust" {
        fn create_profile(name: String, bio: Option<String>) -> UserProfile;
    }
}

// swift
// Create with Some value
let profileWithBio = UserProfile(
    name: "Alice".intoRustString(),
    bio: "Hello world!".intoRustString()
)

// Create with None value  
let profileWithoutBio = UserProfile(
    name: "Bob".intoRustString(),
    bio: nil
)

// Access optional field
if let bio = profileWithBio.bio {
    print("Bio: \(bio.toString())")
}

[Choochmeque]

@chinedufn, please review when you have time.