[google_sign_in] PR 1/4 google_sign_in_ios SPM packaging

[victogomez-cs]

Creates a Swift Package Manager target so later PRs can add Swift sources without mixing them into the Obj-C target. CocoaPods still builds both as one pod.

This is packaging-only: Obj-C sources move to google_sign_in_ios_objc, and a placeholder Swift target re-exports that module. Plugin behavior is unchanged.

Matches the Obj-C / Swift SPM split already used by video_player_avfoundation and in_app_purchase_storekit:

• Package.swift: google_sign_in_ios depends on google_sign_in_ios_objc; only the Swift product is exported
• CocoaPods: combined source_files glob plus Swift stdlib xcconfig
• Header search path stays include/google_sign_in_ios so existing #import <google_sign_in_ios/...> keeps working
• Tests: #if canImport(google_sign_in_ios_objc) for the Obj-C module

Bumps google_sign_in_ios to 6.3.2.

PR 1/4 of the Obj-C → Swift migration. Follows #12484 (coverage backfill). Part of flutter/flutter#119103

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Code Review

This pull request updates the google_sign_in_ios package to version 6.3.2, introducing a Swift Package Manager target for the Swift implementation. It splits the codebase into separate google_sign_in_ios (Swift) and google_sign_in_ios_objc (Objective-C) targets in Package.swift, while combining them in the Podspec for CocoaPods compatibility. Additionally, it updates Pigeon generation paths to output Objective-C files to the new google_sign_in_ios_objc directory and adds conditional imports in tests and a new placeholder file. There are no review comments, so no further feedback is provided.

[auto-submit]

auto label is removed for flutter/packages/12654, Failed to merge flutter/packages/12654 with Pull request flutter/packages/12654 could not be merged: Merging stacked PRs via this endpoint is not supported. Use the asynchronous merge endpoint instead..