add STATIC_LIB option

[valentinstoecker]

The STATIC_LIB optin compiles rayx-core as a static library.
This option is need to compile the python plugin

[Atraxus]

We define options for compiling the code in the root CMakeLists.txt file (e.g. WERROR, RAYX_ENABLE_CUDA). RAYX_STATIC_LIB should be an option there too. You can then simply use -DRAYX_STATIC_LIB=YES to set it during CMake configuration. In CMake you can then use

if(RAYX_STATIC_LIB) 
    ....
else()
    ....
endif()