## What changes were proposed in this pull request?
This PR proposes to make `DataFrameReader.jdbc` call `DataFrameReader.format("jdbc").load` consistently with other APIs in `DataFrameReader`/`DataFrameWriter` and avoid calling `sparkSession.baseRelationToDataFrame(..)` here and there.
The changes were mostly copied from `DataFrameWriter.jdbc()` which was recently updated.
```diff
- val params = extraOptions.toMap ++ connectionProperties.asScala.toMap
- val options = new JDBCOptions(url, table, params)
- val relation = JDBCRelation(parts, options)(sparkSession)
- sparkSession.baseRelationToDataFrame(relation)
+ this.extraOptions = this.extraOptions ++ connectionProperties.asScala
+ // explicit url and dbtable should override all
+ this.extraOptions += ("url" -> url, "dbtable" -> table)
+ format("jdbc").load()
```
## How was this patch tested?
Existing tests should cover this.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#15499 from HyukjinKwon/SPARK-17955.
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