[SPARK-20090][PYTHON] Add StructType.fieldNames in PySpark

## What changes were proposed in this pull request?

This PR proposes `StructType.fieldNames` that returns a copy of a field name list rather than a (undocumented) `StructType.names`.

There are two points here:

  - API consistency with Scala/Java

  - Provide a safe way to get the field names. Manipulating these might cause unexpected behaviour as below:

    ```python
    from pyspark.sql.types import *

    struct = StructType([StructField("f1", StringType(), True)])
    names = struct.names
    del names[0]
    spark.createDataFrame([{"f1": 1}], struct).show()
    ```

    ```
    ...
    java.lang.IllegalStateException: Input row doesn't have expected number of values required by the schema. 1 fields are required while 0 values are provided.
    	at org.apache.spark.sql.execution.python.EvaluatePython$.fromJava(EvaluatePython.scala:138)
    	at org.apache.spark.sql.SparkSession$$anonfun$6.apply(SparkSession.scala:741)
    	at org.apache.spark.sql.SparkSession$$anonfun$6.apply(SparkSession.scala:741)
    ...
    ```

## How was this patch tested?

Added tests in `python/pyspark/sql/tests.py`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18618 from HyukjinKwon/SPARK-20090.