refactor(core,apple): migrate iOS/macOS plugin implementation to Swift

[SelaseKay]

Description

Migrates firebase_core on iOS and macOS from the Pigeon Objective-C generator (objcOut) to Swift (swiftOut), matching recent plugin migrations such as Crashlytics.

The HostApi implementation is now Swift (FLTFirebaseCorePlugin), still registered as pluginClass: FLTFirebaseCorePlugin. Shared ObjC helpers (FLTFirebasePlugin / FLTFirebasePluginRegistry) stay ObjC in a separate firebase_core_objc SPM target because SPM cannot mix Swift and ObjC in one target.

FirebaseOptions.authDomain is not a native FirebaseApp field, so custom auth domains are stored on FLTFirebasePlugin (keyed by iOS app name) instead of the Core plugin class. firebase_auth now reads [FLTFirebasePlugin getCustomDomain:] and no longer imports FLTFirebaseCorePlugin.h.

Android is unchanged in this PR (Java HostApi + javaOut remain). A follow-up Kotlin PR will stack on this branch because both migrations edit pigeons/messages.dart.

Dart unit tests for firebase_core and firebase_core_platform_interface passed locally. Native iOS/macOS builds and melos run analyze were not run in this change.

Related Issues

None

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

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