Avro adapter: setPosition doesn't work with BaseVariableWidthVectors

### Describe the bug, including details regarding any error messages, version, and platform.

`BaseVariableWidthVector`-based vectors (`VarBinaryVector`, `VarCharVector`) expect the data to be written sequentially. This means `Consumer.setPosition` is not safe to use. Imagine the following scenario:

- write index 0
- write index 2
- write index 1
- write index 3

On the last step, `BaseVariableWidthVector.fillHoles` erases the data written for the index 2:

```java
protected final void fillHoles(int index) {
for (int i = lastSet + 1; i < index; i++) {
setBytes(i, emptyByteArray, 0, emptyByteArray.length);
}
lastSet = index - 1;
}
```

Here, index would be 3, lastSet + 1 would be 2 (1 + 1), so `setBytes(2, emptyByteArray, ...)` is called.

Not sure what the right solution is here, but at a minimum, the library shouldn't allow calling `setPosition` for Consumers using `BaseVariableWidthVector` vectors.

首先，追蹤 Avro 配接器在使用以 BaseVariableWidthVector 為基礎的 VarBinaryVector 和 VarCharVector 實例時 Consumer.setPosition 的行為。重現寫入序列 0、2、1、3 並新增回歸測試；當定位不再清除先前寫入的資料，或不再阻止對這些向量進行定位時，即表示完成。

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