feat: Support for building as a vendored library

[Davide-1998]

I have the need to include CycloneDDS as part of my main project, without building it in advance and using it as an imported CMake target, e.g. add_subdirectory(CycloneDDS).
I found out this very challenging because apparently the majority of CMake was structured to not support this feature, e.g. usage of CMAKE_PROJECT_NAME instead of PROJECT_NAME.

[carlocorradini]

Same issue here 😥
Including CycloneDDS with CPM would be AWESOME 🥳