[SPARK-19975][PYTHON][SQL] Add map_keys and map_values functions to Python

## What changes were proposed in this pull request?

This fix tries to address the issue in SPARK-19975 where we
have `map_keys` and `map_values` functions in SQL yet there
is no Python equivalent functions.

This fix adds `map_keys` and `map_values` functions to Python.

## How was this patch tested?

This fix is tested manually (See Python docs for examples).

Author: Yong Tang <yong.tang.github@outlook.com>

Closes #17328 from yongtang/SPARK-19975.