Provide Zserio CMake helper for easier integration to CMake based projects

[Mi-La]

Currently we are working on integration of Zserio to Conan Center Index (#533). We have introduced a cmake helper which could be adopted by the upstream and packed as a part of zserio releases.

It's inspired by protobuf's solution and by zserio-cmake-helper (while it's much simpler for now).

Usage in a CMakeLists.txt (assuming that zserio is installed via conan):

find_package(zserio REQUIRED CONFIG)
add_library(tutorial_zs SHARED tutorial.zs)
zserio_generate_cpp(
    TARGET tutorial_zs
    GEN_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src
)
target_link_libraries(tutorial_zs PUBLIC zserio::ZserioCppRuntime)
target_include_directories(tutorial_zs PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src)

[Mi-La]

zserio_compiler.cmake will be located in release zip for binaries in the cmake subfolder. Documentation is available in README.md for C++ extension or directly in the cmake script.