freertos(IDF): Refactor port heap functions

Vanilla FreeRTOS expects applications to use one of the heap implementations
provided by FreeRTOS (i.e., heap_x.c), where functions such as pvPortMalloc()
and vPortFree() are defined in the heap implementation.

However, ESP-IDF already provides its own heap implementation
(i.e., esp_heap_caps.h). Thus, the pvPortMallc()/vPortFree() functions were
previously overriden by macro to call esp_heap functions directly.

This commit refactors the FreeRTOS port's heap as such:

- Added a heap_idf.c that implements all of the heap related functions required
  by FreeRTOS source
- All dynamic memory allocated by FreeRTOS is from internal memory. Thus, the
  FreeRTOS heap is the internal memory subset of the ESP-IDF heap.
- Removed some old macros to reduce diff from upstream source code.

docs/en/api-reference/system/freertos.rst

@@ -117,3 +117,11 @@ FreeRTOS Additions
 ------------------
 
 ESP-IDF provides some supplemental features to FreeRTOS such as Ring Buffers, ESP-IDF style Tick and Idle Hooks, and TLSP deletion callbacks. See :doc:`freertos_additions` for more details.
+
+FreeRTOS Heap
+-------------
+
+Vanilla FreeRTOS provides its own `selection of heap implementations <https://www.freertos.org/a00111.html>`_. However, ESP-IDF already implements its own heap (see :doc:`/api-reference/system/mem_alloc`), thus ESP-IDF does not make use of the heap implementations provided by Vanilla FreeRTOS. All FreeRTOS ports in ESP-IDF map FreeRTOS memory allocation/free calls (e.g., ``pvPortMalloc()`` and ``pvPortFree()``) to ESP-IDF heap API (i.e., :cpp:func:`heap_caps_malloc` and :cpp:func:`heap_caps_free`). However, the FreeRTOS ports ensure that all dynamic memory allocated by FreeRTOS is placed in internal memory.
+
+.. note::
+    If users wish to place FreeRTOS objects in external memory, users should allocate those objects manually using :cpp:func:`heap_caps_malloc`, then create the object using the object's ``...CreateStatic()`` function.