Add STM32 porting example

[donghoonpark]

STM32 project configuration example to utilize nanomodbus library.

This project contains

• ioc file: hardware driver configuration. can generate driver package and cmake setting with STM32CubeMX
• CMakeLists.txt: nanomodbus & porting source linking procedure with cmake
• nanomodbus_port.c/h: stm32 implementation about transport handling, server data object mapping ...etc
• .vscode/* : vscode project setting as an example

It have been tested with STM32F401CCUx chip with pymodbus client

[donghoonpark]

I've found that nanomodbus cannot support multi-rtu server with current structure. Due to only one address_rtu can be set in nmbs_t. Do you have any plan to support this feature? If you have willing to do it. I'll make pull request including multi-server feature. I will modify my example as single master to avoid confusion.

[donghoonpark]

This example now includes both TCP communication using the W5500 and DMA serial communication on the STM32. The dependency on CubeMX, which was initially included in the pull request, has also been removed, enhancing compatibility so that it can be built with just an appropriate compiler and CMake.