Add thread pool work callbacks API

[sdotpeng]

Description

Add new C API SetGlobalWorkCallbacks to register callbacks for thread pool work lifecycle events:

• on_enqueue - called when work is submitted to the thread pool
• on_start - called when a worker thread begins executing the work
• on_stop - called when work completes (guaranteed even on exception)
  Includes C++ wrapper in Ort::ThreadingOptions and unit tests.

Motivation and Context

Enables external instrumentation and monitoring of thread pool work items (e.g., for profiling, tracing, or custom scheduling insights).

[sdotpeng]

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