[google_sign_in_ios] Adds Swift Package Manager support

[loic-sharma]

Adds Swift Package Manager support to google_sign_in_ios.

This does not migrate the example app's Xcode project to use Swift Package Manager, that's tracked by flutter/flutter#148021.

Fixes flutter/flutter#146904

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• 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 updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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

[loic-sharma]

These lines inhibit warnings but also add AppAuth and GTMAppAuth as CocoaPod dependencies:

pod 'AppAuth', :inhibit_warnings => true
pod 'GTMAppAuth', :inhibit_warnings => true

If the SwiftPM feature is enabled, this caused duplicate modules and the build would fail.

This is a temporary workaround. In the future, we'll remove CocoaPod integration from these example apps. That's tracked by flutter/flutter#148021.

[loic-sharma]

This is bumped up to macOS 10.15. Required to depend on GoogleSignIn (here)

[stuartmorgan]

We already require 10.15 (which is good, because otherwise this would be very hard to roll out in a non-disruptive way).

[loic-sharma]

This mirrors:

packages/packages/google_sign_in/google_sign_in_ios/darwin/google_sign_in_ios.podspec

Lines 20 to 24 in 275e985

	# AppAuth and GTMSessionFetcher are GoogleSignIn transitive dependencies.
	# Depend on versions which defines modules.
	s.dependency 'AppAuth', '>= 1.7.6'
	s.dependency 'GTMSessionFetcher', '>= 3.4.0'
	s.dependency 'GoogleSignIn', '~> 7.1'

[stuartmorgan]

From triage: still blocked on upstream issues.

[loic-sharma]

AppAuth-iOS 1.7.6 was released with the fix, this PR should be unblocked now: openid/AppAuth-iOS#871

[loic-sharma]

This is failing xcode-analyze and native-test because our transitive dependency AppAuth has warnings, and we treat warnings as errors. This isn't a problem when using CocoaPods as we've configured it to ignore these warnings. Unfortunately, SwiftPM doesn't let suppress warnings in our dependencies.

Our options are to either allow packages to opt-out of warnings as errors, or, fix the warnings in the upstream dependency: openid/AppAuth-iOS#888

[stuartmorgan]

I'm okay with temporarily turning off warnings-as-errors for this package, but we should have a tracking issue with context for turning it back on.

Ideally we should also reference an SPM feature request for the ability to do what we used to do with Cocoapods. Having the ability to use different warning settings for code you control and code you don't is a fairly common feature in build systems.