Expose libmodbus's flush command

[pomplesiegel]

• Expose libmodbus's flush command to ModbusRTUServer API
• This command proved to be necessary for stable operation for large reads at low baud rates on the RP2040. May be important for cross-platform compatibility

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Memory usage change @ 5c93b089e99b178ae4af0c90917feb640142d51d

Board	flash	%	RAM for global variables	%
arduino:mbed_nano:nano33ble	:small_red_triangle: 0 - +8	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_opta:opta	:small_red_triangle: 0 - +72	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A
arduino:megaavr:uno2018:mode=off	:small_red_triangle: +12 - +12	+0.02 - +0.02	0 - 0	0.0 - 0.0
arduino:samd:mkrwifi1010	:small_red_triangle: +8 - +16	0.0 - +0.01	0 - 0	0.0 - 0.0

Click for full report table

Board|`examples/RTU/ModbusRTUClientKitchenSink`
flash|%|`examples/RTU/ModbusRTUClientKitchenSink`
RAM for global variables|%|`examples/RTU/ModbusRTUClientParameters`
flash|%|`examples/RTU/ModbusRTUClientParameters`
RAM for global variables|%|`examples/RTU/ModbusRTUClientToggle`
flash|%|`examples/RTU/ModbusRTUClientToggle`
RAM for global variables|%|`examples/RTU/ModbusRTUServerKitchenSink`
flash|%|`examples/RTU/ModbusRTUServerKitchenSink`
RAM for global variables|%|`examples/RTU/ModbusRTUServerLED`
flash|%|`examples/RTU/ModbusRTUServerLED`
RAM for global variables|%|`examples/RTU/ModbusRTUTemperatureSensor`
flash|%|`examples/RTU/ModbusRTUTemperatureSensor`
RAM for global variables|%|`examples/TCP/EthernetModbusClientToggle`
flash|%|`examples/TCP/EthernetModbusClientToggle`
RAM for global variables|%|`examples/TCP/EthernetModbusServerLED`
flash|%|`examples/TCP/EthernetModbusServerLED`
RAM for global variables|%|`examples/TCP/WiFiModbusClientToggle`
flash|%|`examples/TCP/WiFiModbusClientToggle`
RAM for global variables|%|`examples/TCP/WiFiModbusServerLED`
flash|%|`examples/TCP/WiFiModbusServerLED`
RAM for global variables|% -|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|- `arduino:mbed_nano:nano33ble`|0|0.0|0|0.0|8|0.0|0|0.0|0|0.0|0|0.0|8|0.0|0|0.0|8|0.0|0|0.0|8|0.0|0|0.0 `arduino:mbed_opta:opta`|0|0.0|0|0.0|72|0.0|0|0.0|0|0.0|0|0.0|8|0.0|0|0.0|8|0.0|0|0.0|8|0.0|0|0.0|0|0.0|0|0.0|8|0.0|0|0.0 `arduino:mbed_portenta:envie_m7`|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A|||||||| `arduino:megaavr:uno2018:mode=off`|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0|12|0.02|0|0.0 `arduino:samd:mkrwifi1010`|8|0.0|0|0.0|8|0.0|0|0.0|16|0.01|0|0.0|8|0.0|0|0.0|8|0.0|0|0.0|16|0.01|0|0.0|8|0.0|0|0.0|16|0.01|0|0.0|16|0.01|0|0.0|16|0.01|0|0.0

Click for full report CSV

``` Board,examples/RTU/ModbusRTUClientKitchenSink
flash,%,examples/RTU/ModbusRTUClientKitchenSink
RAM for global variables,%,examples/RTU/ModbusRTUClientParameters
flash,%,examples/RTU/ModbusRTUClientParameters
RAM for global variables,%,examples/RTU/ModbusRTUClientToggle
flash,%,examples/RTU/ModbusRTUClientToggle
RAM for global variables,%,examples/RTU/ModbusRTUServerKitchenSink
flash,%,examples/RTU/ModbusRTUServerKitchenSink
RAM for global variables,%,examples/RTU/ModbusRTUServerLED
flash,%,examples/RTU/ModbusRTUServerLED
RAM for global variables,%,examples/RTU/ModbusRTUTemperatureSensor
flash,%,examples/RTU/ModbusRTUTemperatureSensor
RAM for global variables,%,examples/TCP/EthernetModbusClientToggle
flash,%,examples/TCP/EthernetModbusClientToggle
RAM for global variables,%,examples/TCP/EthernetModbusServerLED
flash,%,examples/TCP/EthernetModbusServerLED
RAM for global variables,%,examples/TCP/WiFiModbusClientToggle
flash,%,examples/TCP/WiFiModbusClientToggle
RAM for global variables,%,examples/TCP/WiFiModbusServerLED
flash,%,examples/TCP/WiFiModbusServerLED
RAM for global variables,% arduino:mbed_nano:nano33ble,0,0.0,0,0.0,8,0.0,0,0.0,0,0.0,0,0.0,8,0.0,0,0.0,8,0.0,0,0.0,8,0.0,0,0.0 arduino:mbed_opta:opta,0,0.0,0,0.0,72,0.0,0,0.0,0,0.0,0,0.0,8,0.0,0,0.0,8,0.0,0,0.0,8,0.0,0,0.0,0,0.0,0,0.0,8,0.0,0,0.0 arduino:mbed_portenta:envie_m7,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,,,,,,,, arduino:megaavr:uno2018:mode=off,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0,12,0.02,0,0.0 arduino:samd:mkrwifi1010,8,0.0,0,0.0,8,0.0,0,0.0,16,0.01,0,0.0,8,0.0,0,0.0,8,0.0,0,0.0,16,0.01,0,0.0,8,0.0,0,0.0,16,0.01,0,0.0,16,0.01,0,0.0,16,0.01,0,0.0 ```