[Java] Add API for getBufferSizeFor() with density to BaseVariableWidthVector

Following the discussion on https://github.com/apache/arrow/pull/9187.

Proposed API in BaseVariableWidthVector.java:

```java

/**
* Get the potential buffer size for a particular number of records and density.
* @param valueCount desired number of elements in the vector
* @param density average number of bytes per variable width element
* @return estimated size of underlying buffers if the vector holds
* a given number of elements
*/
public int getBufferSizeFor(final int valueCount, double density)
```

The current `getBufferSizeFor(int valueCount)` for BaseVariableWidthVector requires that validity and offset vectors have already been allocated for at least the given `valueCount`. If the aim of this method is to estimate memory usage for a value count, it's not very useful because it can only give sizes for less than or equal value counts in the currently allocated vector.

A better approach for approximating memory usage is to include a density argument, along with value count. Then the buffer estimate does not require the validity and offset vector to have any allocation. This also is inline with `setInitialCapacity(int valueCount, double density)`

NOTE: this API should also be added to BaseLargeVariableWidthVector and possibly BaseRepeatedValueVector(Large) as well.

**Reporter**: [Bryan Cutler](https://issues.apache.org/jira/browse/ARROW-11739) / @BryanCutler

_{**Note**: *This issue was originally created as [ARROW-11739](https://issues.apache.org/jira/browse/ARROW-11739). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Bắt đầu với BaseVariableWidthVector.java và so sánh getBufferSizeFor(int valueCount) hiện có với setInitialCapacity(int valueCount, double density). Xem lại phần thảo luận của pull request được liên kết, sau đó kiểm tra xem API tương tự có áp dụng cho BaseLargeVariableWidthVector và BaseRepeatedValueVector(Large) hay không; công việc được xem là hoàn tất khi API dựa trên density được yêu cầu được định nghĩa nhất quán trên tất cả các lớp vector phù hợp.

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