Add configuration for rx buffer size in Modbus RTU server

[batt]

I'm using this library as a Modbus RTU server on a Arduino Nano board.

Since I have a lot of other libraries, RAM usage is very high and the application crashes due to stack overflow.

I noticed that inside ModbusRTUServerClass:Poll(), a request buffer is allocated:

void ModbusRTUServerClass::poll()
{
  uint8_t request[MODBUS_RTU_MAX_ADU_LENGTH];

  int requestLength = modbus_receive(_mb, request);

  if (requestLength > 0) {
    modbus_reply(_mb, request, requestLength, &_mbMapping);
  }
}

MODBUS_RTU_MAX_ADU_LENGTH is 256 bytes, which is more than 10% of the RAM available on the Arduino Nano.

Since all Modbus messages in my application are way smaller than that, I added a parameter to the class beginmethod in order to choose the request buffer size.

Do you think that could be useful? I can open a PR if you think so.