[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.

python/pyspark/sql/functions.py

@@ -1855,6 +1855,46 @@ def sort_array(col, asc=True):
     return Column(sc._jvm.functions.sort_array(_to_java_column(col), asc))
 
 
+@since(2.3)
+def map_keys(col):
+    """
+    Collection function: Returns an unordered array containing the keys of the map.
+
+    :param col: name of column or expression
+
+    >>> from pyspark.sql.functions import map_keys
+    >>> df = spark.sql("SELECT map(1, 'a', 2, 'b') as data")
+    >>> df.select(map_keys("data").alias("keys")).show()
+    +------+
+    |  keys|
+    +------+
+    |[1, 2]|
+    +------+
+    """
+    sc = SparkContext._active_spark_context
+    return Column(sc._jvm.functions.map_keys(_to_java_column(col)))
+
+
+@since(2.3)
+def map_values(col):
+    """
+    Collection function: Returns an unordered array containing the values of the map.
+
+    :param col: name of column or expression
+
+    >>> from pyspark.sql.functions import map_values
+    >>> df = spark.sql("SELECT map(1, 'a', 2, 'b') as data")
+    >>> df.select(map_values("data").alias("values")).show()
+    +------+
+    |values|
+    +------+
+    |[a, b]|
+    +------+
+    """
+    sc = SparkContext._active_spark_context
+    return Column(sc._jvm.functions.map_values(_to_java_column(col)))
+
+
 # ---------------------------- User Defined Function ----------------------------------
 
 def _wrap_function(sc, func, returnType):

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -3161,6 +3161,20 @@ object functions {
    */
   def sort_array(e: Column, asc: Boolean): Column = withExpr { SortArray(e.expr, lit(asc).expr) }
 
+  /**
+   * Returns an unordered array containing the keys of the map.
+   * @group collection_funcs
+   * @since 2.3.0
+   */
+  def map_keys(e: Column): Column = withExpr { MapKeys(e.expr) }
+
+  /**
+   * Returns an unordered array containing the values of the map.
+   * @group collection_funcs
+   * @since 2.3.0
+   */
+  def map_values(e: Column): Column = withExpr { MapValues(e.expr) }
+
   //////////////////////////////////////////////////////////////////////////////////////////////
   //////////////////////////////////////////////////////////////////////////////////////////////