Add traceSTARTING_SCHEDULER tracing hook. (#1082)

Add traceSTARTING_SCHEDULER tracing hook. (#1082)

* Add traceSTARTING_SCHEDULER tracing hook.

Discussed here: https://forums.freertos.org/t/tracing-improvements/20097

This hook enables tracers to run code on startup after all RTOS
resources are created and to detect that the scheduler is starting without
relying on traceENTER/traceEXIT macros.

It also provides tracers access to the task handle of all IDLE tasks,
allowing them to be identified unambiguously and without relying on
INCLUDE_xTaskGetIdleTaskHandle.