Cmake migration

[charlestytler]

Adds cmake build files for the C++ code (under Sources/backend-cpp)

• Main CMakeLists.txt file and CMAKE_SOURCE_DIR is Sources/backend-cpp/.
• All libraries are compiled as static libraries and linked to the main executable in Sources/backend-cpp/StreamdeckInterface/
• The vendored websocketpp library uses dynamic CRT linking so the full project inherits this; therefore Sources/backend-cpp/StreamdeckInterface/CMakeLists.txt also copies all necessary DLLs to package with compiled executable to make it portable.

Can be compiled and also tested via:

cmake -B build
cmake --build build --config Release
ctest --test-dir build --output-on-failure

Also updates Tools/build_plugin.bat and github action: cpp_tests.yml to build C++ with cmake instead of VS .sln.

[charlestytler]

This file was copied from a top level Test directory, backend-cpp/Test/MockESDConnectionManager.h. Original file will be deleted later.

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ckend-cpp/ElgatoSD/test/MockESDConnectionManager.h	45.45%	18 Missing ⚠️

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