[SPARK-14555] First cut of Python API for Structured Streaming

## What changes were proposed in this pull request?

This patch provides a first cut of python APIs for structured streaming. This PR provides the new classes:
 - ContinuousQuery
 - Trigger
 - ProcessingTime
in pyspark under `pyspark.sql.streaming`.

In addition, it contains the new methods added under:
 -  `DataFrameWriter`
     a) `startStream`
     b) `trigger`
     c) `queryName`

 -  `DataFrameReader`
     a) `stream`

 - `DataFrame`
    a) `isStreaming`

This PR doesn't contain all methods exposed for `ContinuousQuery`, for example:
 - `exception`
 - `sourceStatuses`
 - `sinkStatus`

They may be added in a follow up.

This PR also contains some very minor doc fixes in the Scala side.

## How was this patch tested?

Python doc tests

TODO:
 - [ ] verify Python docs look good

Author: Burak Yavuz <brkyvz@gmail.com>
Author: Burak Yavuz <burak@databricks.com>

Closes #12320 from brkyvz/stream-python.

1 files changed, 2 insertions, 2 deletions

diff --git a/python/pyspark/ml/feature.py b/python/pyspark/ml/feature.py
index 4310f154b5..a1911cebe3 100644
--- a/python/pyspark/ml/feature.py
+++ b/python/pyspark/ml/feature.py
@@ -21,10 +21,10 @@ if sys.version > '3':
 
 from py4j.java_collections import JavaArray
 
-from pyspark import since
+from pyspark import since, keyword_only
 from pyspark.rdd import ignore_unicode_prefix
 from pyspark.ml.param.shared import *
-from pyspark.ml.util import keyword_only, JavaMLReadable, JavaMLWritable
+from pyspark.ml.util import JavaMLReadable, JavaMLWritable
 from pyspark.ml.wrapper import JavaEstimator, JavaModel, JavaTransformer, _jvm
 from pyspark.mllib.common import inherit_doc
 from pyspark.mllib.linalg import _convert_to_vector