Fix build with Ethernet enabled, simplify CMakeLists.txt, add Github workflow file

[igrr]

Hello @Pro, thank you for maintaining this useful project!

Please consider the following minor fixes and improvements:

• Fix building with Ethernet enabled (CONFIG_ETHERNET_HELPER_ETHERNET=y)
• Pass CMAKE_TOOLCHAIN_FILE to open62541 subproject, which enables building even if no C host compiler is installed. Likely fixes issue when building my esp32 project! can anybody help? #6, Performing configure step for 'open62541_build' FAILED #7.
• Use CMake GENERATED property instead of creating source files manually in open62541 component CMakeLists.txt file.
• Select minimum required flash size (4MB) by default in sdkconfig.defaults, saving one step for users when configuring the project.
• Add Github Actions workflow file to verify that project builds with multiple IDF release branches (release/v4.1-release/v4.3). Example workflow run: https://github.com/igrr/open62541-esp32/actions/runs/796549763.

[Pro]

Nice, thank you very very much for this nice addition!