Add python bindings to the global thread pool functionality

[khoover]

Description

Allows users to configure and enable the global thread pool via Python, and have inference sessions use it instead of session-local thread pools.

TODO: exact changes

Motivation and Context

Forked off of #23495 to take over implementation, see issue #23523.

Our particular use case involves a single service instance serving thousands of individual models, each relatively small (e.g. small decision trees). Creating individual services for each model is too much overhead, and attempting to start several thousand thread-pools is a non-starter. We could possibly have each session be single-threaded, but we would like to be able to separate the request handler thread count from the compute thread count (e.g. 2 handler threads but 4 intra-op ones).

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[snnn]

We don't have a good way to shutdown the thread pool. It should be done before the python process started to exit and unload DLLs. So this doc: https://learn.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices#deadlocks-caused-by-lock-order-inversion
So, generally speaking, if you have a thread pool and you declared it as a global var, then you must manually shutdown it, otherwise it will cause a deadlock. The issue was highly reproducible. However, recently probably the restriction is gone? I am not sure. But the Windows doc is not updated. So I am not very sure if this will work.

[snnn]

Like this: apache/mxnet#11163

[khoover]

I don't suppose registering a shutdown function via `atexit` would be enough?