iOS OAuth with PKCE example client

This is a very small SwiftUI app that demonstrates how to authenticate using OAuth using the authorization code grant with PKCE.

It is designed to work with the proof-of-concept PKCE proxy server.

The example app does not use any third-party libraries. It:

• Uses ASWebAuthenticationSession from the AuthenticationServices framework to allow the user to authenticate with a third-party service via the PKCE proxy and obtain the authorization grant code.
• Uses CryptoKit to generate the PKCE code challenge.
• Makes a simple POST request using URLSession to exchange the authentication code for an access token, again via the PKCE proxy.

This is not production-ready code and is simply a proof of concept.