[MINOR][SQL] Add missing functions for some options in SQLConf and use them where applicable

## What changes were proposed in this pull request?

I first thought they are missing because they are kind of hidden options but it seems they are just missing.

For example, `spark.sql.parquet.mergeSchema` is documented in [sql-programming-guide.md](https://github.com/apache/spark/blob/master/docs/sql-programming-guide.md) but this function is missing whereas many options such as `spark.sql.join.preferSortMergeJoin` are not documented but have its own function individually.

So, this PR suggests making them consistent by adding the missing functions for some options in `SQLConf` and use them where applicable, in order to make them more readable.

## How was this patch tested?

Existing tests should cover this.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #14678 from HyukjinKwon/sqlconf-cleanup.