feat: add first protocol event

[kieran-osgood-shopify]

What changes are you making?

Subscribing to events should look like this

  <CheckoutSheet
        url={checkoutUrl}
        onCancel={() => setVisible(false)}
        onFail={() => {}}
        protocol={{
          start:    chk => analytics.track('checkout_start', { id: chk.id }),
          complete: chk => {
            reset();
            setVisible(false);
          },
          error:    err => toast(err.messages[0]?.content ?? 'Checkout error'),
        }}
      />


 ShopifyCheckoutKit.present(url, {onFail: () => {}, onCancel: () => {}}, { [CheckoutProtocol.start]: c => {c.checkourUrl} })

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 relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)

---

Releasing a new Swift version?

• I have bumped the version in ShopifyCheckoutKit.podspec
• I have bumped the version in platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
• I have updated platforms/swift/CHANGELOG.md
• I have updated the SwiftPM/CocoaPods version snippets in platforms/swift/README.md (major version only)

Releasing a new Android version?

• I have bumped the versionName in platforms/android/lib/build.gradle
• I have updated platforms/android/CHANGELOG.md
• I have updated the Gradle/Maven version snippets in platforms/android/README.md

Tip

See the Contributing documentation for the full release process per platform.

[kieran-osgood-shopify]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: add first protocol event #134 👈 (View in Graphite)
• feat: remove addEventListener API #119 : 2 other dependent PRs (#125 , #170 )
• feat(react-native): use local android/swift project references #115 : 1 other dependent PR (#116 )
• feat(protocol): add typescript interfaces #114
• main

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