[Improvement] Reorganize the code directory to make developer easily to extend library for other language

[acezen]

Is your feature request related to a problem? Please describe.
Since the C++ library is the first library support by GraphAr, its code is put directly in the root of source. For extending other language library easily, we need to reorganize the code directory like:

.
├── cpp (c++ library code)
├── docs
├── examples
├── spark
└── thirdparty

Describe the solution you'd like

• put the c++ library code to cpp directory
• Add an CMakeList.txt to manage the building of all libraries