Implement iterator for StructArray

Right now if you were to use the C++ library with nanoarrow and read in a stream of two dimensional objects, you would:

1. Use the `ViewArrayStream` class to iterate the stream
2. Manually loop over each schema / array in the StructArray
3. Use the `ViewArrayAs` class to iterate the individual array views

Not sure if this falls in the scope of nanoarrow or if its something for sparrow, but I think it would make sense to add an iterator for step 2

Start by reading ViewArrayStream and ViewArrayAs usage around StructArray handling, then compare whether the iterator belongs in nanoarrow or sparrow. Done means callers can iterate the individual arrays in a StructArray without manually looping over its schema and array components.

Written by the indexing model from the issue text.