nvs::NVSHandle::get_used_entry_count() method

- ESP_OK if the changes have been written successfully. Return param used_entries will be filled valid value. - ESP_ERR_NVS_NOT_INITIALIZED if the storage driver is not initialized. Return param used_entries will be filled 0. - ESP_ERR_INVALID_ARG if nvs_stats equal to NULL. - Other error codes from the underlying storage driver. Return param used_entries will be filled 0.