[SPARK-19795][SPARKR] add column functions to_json, from_json

## What changes were proposed in this pull request?

Add column functions: to_json, from_json, and tests covering error cases.

## How was this patch tested?

unit tests, manual

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #17134 from felixcheung/rtojson.