[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

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

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.