fix(auth): make AuthClient an Actor

[grdsdev]

This pull request includes significant changes to the AuthClient class in Sources/Auth/AuthClient.swift to improve concurrency handling and integrate Combine for handling app lifecycle changes.

Concurrency improvements:

• Changed AuthClient from a final class to an actor to leverage Swift's concurrency model.

To keep backward compatibility, had to mark a few properties and methods of AuthClient as nonisolated, the nonisolated mark, shall be removed on a next major release.

Combine integration:

Because of the change from class to actor, I had to change the way we observe the notification center.

• Added Combine import conditionally based on availability.
• Refactored observeAppLifecycleChanges to use Combine publishers for handling app lifecycle notifications.

These changes improve the concurrency model of AuthClient and modernize the handling of app lifecycle events using Combine.

[coveralls]

Pull Request Test Coverage Report for Build 13312951371

Details

• 46 of 62 (74.19%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 74.38%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Sources/Auth/AuthClient.swift	46	62	74.19%

Files with Coverage Reduction	New Missed Lines	%
Sources/Auth/AuthClient.swift	2	88.5%

Totals
Change from base Build 13261463260:	-0.1%
Covered Lines:	5011
Relevant Lines:	6737

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💛 - Coveralls