[SPARK-4561] [PYSPARK] [SQL] turn Row into dict recursively

Add an option `recursive` to `Row.asDict()`, when True (default is False), it will convert the nested Row into dict.

Author: Davies Liu <davies@databricks.com>

Closes #8006 from davies/as_dict and squashes the following commits:

922cc5a [Davies Liu] turn Row into dict recursively