[SPARK-13479][SQL][PYTHON] Added Python API for approxQuantile

## What changes were proposed in this pull request?

* Scala DataFrameStatFunctions: Added version of approxQuantile taking a List instead of an Array, for Python compatbility
* Python DataFrame and DataFrameStatFunctions: Added approxQuantile

## How was this patch tested?

* unit test in sql/tests.py

Documentation was copied from the existing approxQuantile exactly.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #11356 from jkbradley/approx-quantile-python.