[Feat][C++] Unifying namespace and designing more OOP

[Thespica]

Is your feature request related to a problem? Please describe.
This issue is about designing of namespace. It's really confusing that there are two similar namespace called utils and util, but both are under subdirectory and header called utils.

https://github.com/alibaba/GraphAr/blob/3cd7038cd884bd387d9d005b3165118a6fcfcd31/cpp/include/gar/utils/utils.h#L41

https://github.com/alibaba/GraphAr/blob/3cd7038cd884bd387d9d005b3165118a6fcfcd31/cpp/include/gar/utils/reader_utils.h#L29C18-L29C18

Describe the solution you'd like

1. Unify name of directories, headers and namespaces to utils or util.

2. Designing more OOP, reduce functions beyond classes: It will be easier to call C++ functions in Java if we put functions(static or not) in classes rather than in namespaces, because functions must rely on class.

Good example:

https://github.com/alibaba/GraphAr/blob/3cd7038cd884bd387d9d005b3165118a6fcfcd31/cpp/include/gar/graph_info.h#L1091

Bad example:

https://github.com/alibaba/GraphAr/blob/3cd7038cd884bd387d9d005b3165118a6fcfcd31/cpp/include/gar/utils/reader_utils.h#L46

Maybe we can keep origin functions and copy needed to classes.