feat(nvs_flash): Added Kconfig option contolling NVS heap allocation source

NVS configuration is extended with Kconfig option controlling RAM area for
NVS heap allocation. Either Internal RAM or SPIRAM can be chosen.
Tests were extended to check memory consumption from Internal and SPIRAM pool with respect
to the Kconfig option chosen.
Documentation was extended with notes related to NVS behavior in various situations.

components/nvs_flash/test_apps/pytest_nvs_flash.py

@@ -1,6 +1,5 @@
 # SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
 # SPDX-License-Identifier: Apache-2.0
-
 import pytest
 from pytest_embedded_idf.dut import IdfDut
 
@@ -39,3 +38,9 @@ def test_nvs_flash_encr_flash_enc(dut: IdfDut) -> None:
     # Erase the nvs_key partition
     dut.serial.erase_partition('nvs_key')
     dut.run_all_single_board_cases()
+
+
+@pytest.mark.esp32
+@pytest.mark.psram
+def test_nvs_flash_ram(dut: IdfDut) -> None:
+    dut.run_all_single_board_cases(group='nvs_ram')