esp_ble_gattc_send_mtu_req() function

- ESP_OK: Success - ESP_FAIL: Failure

1. This function triggers `ESP_GATTC_CFG_MTU_EVT`. 2. You could call `esp_ble_gatt_set_local_mtu` to set the desired MTU size locally before this API. If not set, the GATT channel uses the default MTU size (23 bytes).