[SPARK-13842] [PYSPARK] pyspark.sql.types.StructType accessor enhancements

## What changes were proposed in this pull request?

Expand the possible ways to interact with the contents of a `pyspark.sql.types.StructType` instance.
  - Iterating a `StructType` will iterate its fields
    - `[field.name for field in my_structtype]`
  - Indexing with a string will return a field by name
    - `my_structtype['my_field_name']`
  - Indexing with an integer will return a field by position
    - `my_structtype[0]`
  - Indexing with a slice will return a new `StructType` with just the chosen fields:
    - `my_structtype[1:3]`
  - The length is the number of fields (should also provide "truthiness" for free)
    - `len(my_structtype) == 2`

## How was this patch tested?

Extended the unit test coverage in the accompanying `tests.py`.

Author: Sheamus K. Parkes <shea.parkes@milliman.com>

Closes #12251 from skparkes/pyspark-structtype-enhance.