Swift package traits in native integration

Tuist projects that use Xcode's native package integration can now select Swift package traits directly in Project.packages:

let project = Project(
    name: "App",
    packages: [
        .package(path: "Package", traits: ["NativeIntegration"]),
    ]
)

The project description interface follows Swift Package Manager's Package.Dependency model. Traits are typed values, .defaults enables the package's default traits, and an empty set disables them. Package traits require Xcode 26.4 or later, and existing package declarations continue to work through the deprecated compatibility initializer.