refactor(freertos): Remove option for Task Snapshot

Previously, Task Snapshot could be conditionally built based on the
CONFIG_FREERTOS_ENABLE_TASK_SNAPSHOT option. However, there is no overhead
with always enabling Task Snapshot functions, as the linker will remove any
functions if they are not called.

This commit...

- removes all ESP-IDF dependencies on the CONFIG_FREERTOS_ENABLE_TASK_SNAPSHOT
option so that Task Snapshot functions are always available.
- Makes CONFIG_FREERTOS_ENABLE_TASK_SNAPSHOT a hidden option to maintain
compatibility with user code.
- adds documentation regarding Task Snapshot.

components/freertos/test_apps/freertos/port/test_tasks_snapshot.c

@@ -5,8 +5,6 @@
  */
 
 #include "sdkconfig.h"
-
-#if CONFIG_FREERTOS_ENABLE_TASK_SNAPSHOT
 #include <stdio.h>
 #include <stdbool.h>
 #include "freertos/FreeRTOS.h"
@@ -187,5 +185,3 @@ TEST_CASE("Task snapshot: Iterate", "[freertos]")
     check_snapshots(task_list, num_tasks, task_snapshots, num_snapshots);
     teardown(task_list, num_tasks, old_priority);
 }
-
-#endif // CONFIG_FREERTOS_ENABLE_TASK_SNAPSHOT