[Java] Represent a data element of a vector as a tree of ArrowBufPointer

For a fixed/variable width vector, each of its data element can be represented as an ArrowBufPointer object, which represents a contiguous memory segment. This makes many tasks easier and more efficient (without memory copy): calculating hash code, comparing values, etc.

This cannot be achieved for complex vectors, because their values often reside in more than one contiguous memory regions. However, it can be seen that the contiguous memory regions for each data element forms a tree-like structure, whose leaf nodes are the contiguous memory regions. For example, a data element for a struct vector forms a tree, whose root corresponds to the struct vector, while the child vectors corresponds to the child nodes of the tree root.

In this issue, we provide a data structure that represents each data element of a vector as a tree, whose leaf nodes are ArrowBufPointers, representing contiguous memory regions for the data element.

With this data structure, many tasks also becomes easier and more efficient: calculating hash code, comparing vector elements (ordering & equality). In addition, we can do something that could not have been done in the past, like placing data elements into a hash table/hash set, etc.

**Reporter**: [Liya Fan](https://issues.apache.org/jira/browse/ARROW-7213) / @liyafan82
#### PRs and other links:
- [GitHub Pull Request apache/arrow#5883](https://github.com/apache/arrow/pull/5883)

_{**Note**: *This issue was originally created as [ARROW-7213](https://issues.apache.org/jira/browse/ARROW-7213). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Bắt đầu với mô tả gốc của ARROW-7213 và GitHub Pull Request #5883, là những tham chiếu triển khai duy nhất của issue. Xác định các kiểu vector, API và hành vi được đề cập ở đó; công việc được hoàn thành khi tồn tại một biểu diễn cây có thể sử dụng cho các phần tử vector và hỗ trợ các thao tác tiếp theo đã nêu mà không sao chép không cần thiết.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.