[VL] Build: Import Velox C++ as a child CMake project

Currently Velox C++ library is [statically imported](https://github.com/apache/incubator-gluten/blob/e436fa4899eee052a5ed019542d8ce5a4183ffd5/cpp/velox/CMakeLists.txt#L216-L217) into Velox backend's C++ code. Which means, we should manage all the trivial details for building Velox, especially, we had to import all the 3rd libraries that are required by Velox by hand. This creates unnecessary burden on development from Gluten side.

To improve this part, we can import Velox's C++ code as a child CMake project by `FetchContent` in Gluten's CMake script, to keep all the CMake contexts from Velox. Thus, 3rd dependency management will be made much easier because the majority of it was already handled by Velox's build process.