Add nanosleep function

[David-LP99]

Description

ESP-IDF defines _POSIX_TIMERS as 1 and provides several POSIX time functions, but nanosleep() is not implemented by ESP-IDF or by the Newlib/Picolibc archives shipped with the toolchain.

This causes portable code which uses nanosleep(), directly or through libstdc++, to fail at link time with:

    undefined reference to `nanosleep'

ros2/rclc uses the function usleep() to implement rclc_sleep_ms(), but this function has been deprecated from POSIX since 2008, and GCC 14 or higher raise an error because of this. For this reason, ros2/rclc#458 modifies the implementation to use nanosleep() instead.

Main changes

• Added a new file with the nanosleep() implementation
• Modified CMakeLists.txt to include the change

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