esp_ble_gatts_app_register() function

- ESP_OK: Success - ESP_ERR_INVALID_ARG: The input `app_id` exceeds `ESP_APP_ID_MAX` (0x7fff) defined in esp_bt_defs.h. - ESP_FAIL: Failure due to other reasons

1. This function triggers `ESP_GATTS_REG_EVT`. 2. The maximum number of applications is limited to 6.