[SPARK-17912] [SQL] Refactor code generation to get data for ColumnVector/ColumnarBatch

## What changes were proposed in this pull request?

This PR refactors the code generation part to get data from `ColumnarVector` and `ColumnarBatch` by using a trait `ColumnarBatchScan` for ease of reuse. This is because this part will be reused by several components (e.g. parquet reader, Dataset.cache, and others) since `ColumnarBatch` will be first citizen.

This PR is a part of https://github.com/apache/spark/pull/15219. In advance, this PR makes the code generation for  `ColumnarVector` and `ColumnarBatch` reuseable as a trait. In general, this is very useful for other components from the reuseability view, too.
## How was this patch tested?

tested existing test suites

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #15467 from kiszk/columnarrefactor.