feat: DO NOT REVIEW CI TEST : not multithread safe access to cancel var

[abelonogov-ld]

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• [ ] I have added test coverage for new or changed functionality
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[!NOTE] Introduce TimeoutExecutor to manage async operations with timeouts and refactor LDClient.identify and LDClient.start to use it; add unit tests.

• Core:
  • Utility: Add ServiceObjects/TimeoutExecutor.swift to run async operations with a timeout, ensuring single completion on a specified DispatchQueue.
  • LDClient:
    • Refactor identify(context:timeout:useCache:completion:) to use TimeoutExecutor instead of manual cancel flags.
    • Refactor timed-start path start(..., startWaitSeconds:, ...) to use TimeoutExecutor for timeout handling.
• Tests:
  • Add LaunchDarklyTests/ServiceObjects/TimeoutExecutorSpec.swift covering result-before-timeout, timeout fallback, race, specified-queue execution, no-completion behavior, and late-result ignored.
• Project:
  • Wire new sources and tests into build targets.

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