[SPARK-20775][SQL] Added scala support from_json

## What changes were proposed in this pull request?

from_json function required to take in a java.util.Hashmap. For other functions, a java wrapper is provided which casts a java hashmap to a scala map. Only a java function is provided in this case, forcing scala users to pass in a java.util.Hashmap.

Added the missing wrapper.

## How was this patch tested?
Added a unit test for passing in a scala map

Author: setjet <rubenljanssen@gmail.com>

Closes #18094 from setjet/spark-20775.