[google_maps_flutter_ios]: Add swift package manager compatibility

[jokerttu]

Makes google_maps_flutter_ios available as a Swift Package to Flutter for iOS 15+ while maintaining compatibility with CocoaPods for iOS 14+.

It follows the same pattern used for other packages in the repository to ensure consistency.

The GoogleMapsUtils library introduces a separate target for Objective-C when using Swift Package Manager (SPM). As a result, package imports are handled differently depending on whether CocoaPods or SPM is used. This is controlled using the FGM_USING_COCOAPODS flag, which is defined in the package's Podfile.

Closes #146920

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.

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[jokerttu]

@cbracken
Swift Package Manager (SPM) works only with iOS15+
But CocoaPods still can be used with iOS14 with older GoogleMaps and GoogleMapsUtils versions.

Seems like the CI runs tests with
flutter config --enable-swift-package-manager
meaning that tests for iOS14 will fail as it GoogleMaps and GoogleMapsUtils for SPM require iOS15.

NOTE: There is two integration test sets, one for iOS14 and one for iOS15,

I did removed 'OCMock' pod only for iOS15 example (and replaced with swift package) as SPM cannot be used with iOS14 example.

TestRunnerUI is not available for iOS15, so these tests are runned only for iOS14 at the moment (with cocoapods packages).

Should iOS14 support be dropped for CocoaPods as well (even when it works?)
(and should there be iOS16 tests as well?)