some initial code for showing authentication with new customer accounts

What changes are you making?

Includes:

launching a login page webview
handling the auth code redirect
completing the OAuth authorization code with pkce exchange

Doesn't yet include:

Storing tokens securely
Refresh token requests
Attaching obtained tokens to cart mutations
Can we render new customer accounts login in dark mode?

(Possibly not all idiomatic swift)

How to test

Before you merge

[!IMPORTANT]

[ ] I've added tests to support my implementation

[ ] I have read and agree with the Contribution Guidelines.

[ ] I have read and agree with the Code of Conduct.

[ ] I've updated the README.

Checklist for releasing a new version

- [ ] I have bumped the version number in the [`podspec` file](https://github.com/Shopify/checkout-kit-swift/blob/main/ShopifyCheckoutKit.podspec#L2). - [ ] I have bumped the version number in the [README](https://github.com/Shopify/checkout-kit-swift/blob/main/README.md#packageswift). - [ ] I have added a [Changelog](https://github.com/shopify/checkout-sheet-kit-swift/blob/main/CHANGELOG.md) entry.

[!TIP] See the Contributing documentation for instructions on how to publish a new version of the library.

Shopify / checkout-sheet-kit-swift