[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.*}

從 BaseVariableWidthVector.java 開始，將現有的 getBufferSizeFor(int valueCount) 與 setInitialCapacity(int valueCount, double density) 進行比較。查看所連結 pull request 的討論，接著確認相同的 API 是否適用於 BaseLargeVariableWidthVector 和 BaseRepeatedValueVector(Large)；當所要求的基於密度的 API 在所有適用的向量類別中都一致定義時，即表示完成。

由索引模型根據 Issue 內容生成。