Swift sample project setup through `example/`

Set up a Swift sample project under the `example/` directory to demonstrate how to integrate and use the FastAPI extension with Apple’s `DeviceCheck` services on iOS.

This example should showcase basic functionality such as obtaining a device token, sending it to the FastAPI backend, and receiving a verification response. The goal is to provide a simple, working reference for developers to get started quickly with the library.

Create a Swift iOS project in the `example/` directory. The example should obtain a device token from Apple's DeviceCheck, send it to a FastAPI backend (likely using the library in this repo), and handle the verification response. Start by reviewing the existing FastAPI extension code to understand the expected API. A working example requires setting up both an iOS app and a local backend server.

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