init: Introduce thread pool

This commit introduces thread pool. Users use Enqueue() to push a task
to the thread pool with its arguments. The thread pool stops when there
is tasks in the queue, there is no running thread, and Wait() is already
called.

Test: atest system/core/init/thread_pool_test.cpp
Bug: 421025789
Change-Id: I466300629f6716267f1ba93326b45e365e092330