[local_auth] Convert iOS/macOS to Swift

[stuartmorgan-g]

Rewrites the iOS/macOS implementation in Swift. This is an in-place rewrite with minimal changes, to minimize the chances of breakage, and to simplify review. Future refactorings can improve the Swift structure (e.g., fully adopting thread enforcement).

https://github.com/flutter/flutter/issues/119104

Pre-Review Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter.
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I linked to at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under[^1].
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under[^1].
• [x] I updated/added any relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under[^1].
• [x] All existing and new tests are passing.

[^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.

[stuartmorgan-g]

Hm, it looks like (any Error) is newer Swift syntax than our version of ~Xcode~ swift-format? I'll look into that and fix it.

[auto-submit[bot]]

autosubmit label was removed for flutter/packages/9459, because - The status or check suite Linux_android android_build_all_packages stable has failed. Please fix the issues identified (or deflake) before re-applying this label.