[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.*}

Comienza con la descripción original de ARROW-7213 y GitHub Pull Request #5883, que son las únicas referencias de implementación del issue. Determina los tipos de vectores, las APIs y el comportamiento que se contemplan allí; el trabajo está terminado cuando existe una representación de árbol utilizable para los elementos vectoriales y admite las operaciones posteriores indicadas sin copias innecesarias.

Escrito por el modelo de indexación a partir del texto del issue.