[Python] Cast one chunk at a time in ChunkedArray ArrowStream export with requested type

[jorisvandenbossche]

Follow-up on #40818, see #40818 (comment) for context.

ChunkedArray.__arrow_c_stream__ currently casts the full ChunkedArray up front when providing a requested_type. Instead, we can also catst one chunk at a time when the stream is being read, to reduce memory consumption.

This is similar to what was implemented for RecordBatchReader in #40070 (CastingRecordBatchReader)

[paleolimbot]

Thanks for opening this...the CastingRecordBatchReader was slightly easier because we already had a RecordBatchReader. I will try to squeeze this one in (unless somebody else gets here first 🙂 ).