Support option to download a private instance of vcpkg and install the necessary ports

[traversaro]

This would drastically simplify the effort necessary to create the installer. The logic to install the port is already in https://github.com/robotology-playground/vcpkg-cpack/blob/master/CMakeLists.txt#L232 , we just need some logic to download the vcpkg repo and compile it (i.e. run the boostrap script).
Probably it is possible to just use the CMake's FetchContent module.