## 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.
e7791c4f69