feat(gen): support Display types for security scopes

[JMLX42]

Summary

This PR implements support for using any type that implements Display for security scopes in #[utoipa::path] and #[openapi] macros, as requested in #1462.

Changes

• Modified SecurityRequirementsAttrItem in security_requirement.rs to use LitStrOrExpr instead of just string literals for scopes
• Updated parsing logic to accept both literals and expressions
• Modified ToTokens implementation to call .to_string() on both literals and expressions to generate proper String values
• Added comprehensive tests for both #[utoipa::path] and #[openapi] macros

Benefits

• Type Safety: Compile-time validation of security scopes
• Better Organization: Scopes can be defined as enums/structs with clear semantics
• IDE Support: Autocomplete and refactoring tools work better with types
• Maintainability: Centralized scope definitions that are easier to update
• Backwards Compatible: String literals still work exactly as before

Example Usage

use std::fmt::Display;

enum Scope {
    Read,
    Write,
}

impl Display for Scope {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Scope::Read => write!(f, "read:items"),
            Scope::Write => write!(f, "write:items"),
        }
    }
}

#[utoipa::path(
    get,
    path = "/items",
    responses(
        (status = 200, description = "success response")
    ),
    security(
        ("oauth2" = [Scope::Read, Scope::Write])
    )
)]
fn get_items() -> String {
    // ...
}

Testing

Added tests demonstrating:

• Using enum types with Display implementation
• Mixing string literals and Display types in the same security requirement
• Backwards compatibility with existing string literal syntax
• Support in both #[utoipa::path] and #[openapi] macros

All existing tests continue to pass without modification.

Closes #1462