[SPARK-22002][SQL] Read JDBC table use custom schema support specify partial fields.

## What changes were proposed in this pull request?

https://github.com/apache/spark/pull/18266 add a new feature to support read JDBC table use custom schema, but we must specify all the fields. For simplicity, this PR support  specify partial fields.

## How was this patch tested?
unit tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #19231 from wangyum/SPARK-22002.