[SPARK-20748][SQL] Add built-in SQL function CH[A]R.

## What changes were proposed in this pull request?
Add built-in SQL function `CH[A]R`:
For `CHR(bigint|double n)`, returns the ASCII character having the binary equivalent to `n`. If n is larger than 256 the result is equivalent to CHR(n % 256)

## How was this patch tested?
unit tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18019 from wangyum/SPARK-20748.