arrow

GH-40066: [Python] Support `requested_schema` in `__arrow_c_stream__()` (#40070)

### Rationale for this change

The `requested_schema` portion of the `__arrow_c_stream__()` protocol methods errored in all cases if passed an unequal schema. There was a note about figuring out how to check the cast before doing it and a comment in https://github.com/apache/arrow/issues/40066 about how it should be done lazily. This PR (hopefully) solves both!

### What changes are included in this PR?

- Added `arrow::py::CastingRecordBatchReader`, which wraps a `arrow::RecordBatchReader`, casting each batch as it is pulled.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes: the current approach adds `RecordBatchReader.cast()` as the way to access the casting reader.

* Closes: #40066
* GitHub Issue: #40066

Lead-authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Co-authored-by: Dewey Dunnington <dewey@voltrondata.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>