feat: upgrade Google iOS SDK to v8

[MalcolmTomisin]

Summary

Upgrades the iOS Google Sign-In SDK from 7.x to 8.x and adds the CocoaPods configuration required by its Swift dependencies.

Changes

• Upgrade GoogleSignIn to ~> 8.0.
• Update the Expo config plugin to add use_modular_headers! to generated iOS Podfiles.
• Make the Podfile modification idempotent.
• Document the required React Native CLI CocoaPods configuration.
• Add tests covering modular-header injection and error handling.

Testing

• Plugin build
• TypeScript checks
• ESLint
• 51 Jest tests
• CocoaPods installation with GoogleSignIn 8.0.0
• Android physical-device build, installation, and launch
• Android button rendering, interactive sign-in, sign-out, fallback, and cancellation
• iOS physical-device build, installation, launch, and JavaScript bundle loading
• Confirmed no missing native module, URL scheme, or React Native SVG errors

Notes

GoogleSignIn 8.x requires iOS 12 or later. The example currently targets iOS 15.1, so this does not raise its effective deployment target.

React Native CLI consumers must add use_modular_headers! to their Podfile. Expo consumers receive this configuration automatically through the package’s config plugin.