Modbus: allows modbus thread stack size customization with MODBUS_STACK_SIZE macro.

For example, add "-DMODBUS_STACK_SIZE=0x2000" to target CFLAGS to limit stack size to 128kB

<?xml version='1.0' encoding='utf-8'?>
<MQTTClient xmlns:xsd="http://www.w3.org/2001/XMLSchema" Broker_URI="ws://172.17.0.1:1883" Use_MQTT_5="true"/>