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.

include/FreeRTOS.h

@@ -621,6 +621,13 @@
     #define traceTASK_SWITCHED_IN()
 #endif
 
+#ifndef traceSTARTING_SCHEDULER
+
+/* Called after all idle tasks and timer task (if enabled) have been created
+ * succesfully, just before the scheduler is started. */
+    #define traceSTARTING_SCHEDULER( xIdleTaskHandles )
+#endif
+
 #ifndef traceINCREASE_TICK_COUNT
 
 /* Called before stepping the tick count after waking from tickless idle

tasks.c

@@ -3735,6 +3735,8 @@ void vTaskStartScheduler( void )
         /* Setting up the timer tick is hardware specific and thus in the
          * portable interface. */
 
+        traceSTARTING_SCHEDULER( xIdleTaskHandles );
+
         /* The return value for xPortStartScheduler is not required
          * hence using a void datatype. */
         ( void ) xPortStartScheduler();