[SPARK-23120][PYSPARK][ML] Add basic PMML export support to PySpark

[holdenk]

What changes were proposed in this pull request?

Adds basic PMML export support for Spark ML stages to PySpark as was previously done in Scala. Includes LinearRegressionModel as the first stage to implement.

How was this patch tested?

Doctest, the main testing work for this is on the Scala side. (TODO holden add the unittest once I finish locally).

[SparkQA]

Test build #89899 has finished for PR 21172 at commit 8cbef82.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• class LinearRegressionModel(JavaModel, JavaPredictionModel, GeneralJavaMLWritable, JavaMLReadable):
• class GeneralMLWriter(MLWriter):
• class GeneralJavaMLWriter(JavaMLWriter):
• class GeneralJavaMLWritable(JavaMLWritable):

[SparkQA]

Test build #89900 has finished for PR 21172 at commit 9b14a1c.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #89902 has finished for PR 21172 at commit 64d6db8.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #90234 has finished for PR 21172 at commit af21aa7.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #90236 has finished for PR 21172 at commit 9d7d684.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #90515 has finished for PR 21172 at commit 67b59a7.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[holdenk]

Merged to master

[vruusmann]

Here's a pointer to another PySpark-to-PMML conversion tool: https://github.com/jpmml/pyspark2pmml