Enable GENAI with FetchContent or find_package() for client projects

[apwojcik]

The PR changes the onnxruntime-genai CMake files so client projects can use it via FetchContent or find_package() CMake commands.

To accomplish that

• the project() needs version configured correctly and up to date,
• the onnxruntime-genai target needs a properly configured INTERFACE when installing and building,
• correctly place binaries and files needed for development in the designated directories,
• export required information and generate config package scripts (CMakeConfigPackageHelpers),

With those changes, it is enough to use find_package() and target_link_libraries() commands in a client project to configure it with the onnxruntime-genai. Please take a look at examples\c\CMakeLists.txt for details, it has been updated accordingly.

A few changes were required for --buld_csharp to accommodate changes from this PR. In addition, the ability to build the Release NuGet package for C# has been added.

A minor optimization for MultiConfig generators is that build.py generates a single build directory rather than one per build type.