Support `on_thread_spawn` and `on_thread_destroy` for `TaskPoolPlugin`

[Logiquo]

Objective

• Allow to configure on_thread_spawn and on_thread_destroy when using TaskPoolPlugin of bevy.

Solution

• In TaskPoolThreadAssignmentPolicy, two options on_thread_spawn and on_thread_destroy are added, which will be passed to two new methods motioned above when creating corresponding task pool using builder.
• Due to lack of debug derive for these two options, manually implement the debug for TaskPoolThreadAssignmentPolicy.

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Changelog

Added

• on_thread_spawn option and on_thread_destroy option to the TaskPoolPlugin, allow user to customize them as needed.

Migration Guide

• TaskPooolThreadAssignmentPolicy now has two additional fields: on_thread_spawn and on_thread_destroy. Please consider defaulting them to None.

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[NthTensor]

Ah of course, don't know how I missed those.