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GH-41749: [GLib] Allow getting a RecordBatchReader from a Dataset or Scanner (#41750)

### Rationale for this change

See #41749

### What changes are included in this PR?

Adds `to_reader` methods to `GADatasetDataset` and `GADatasetScanner`.

### Are these changes tested?

Yes I've added new unit tests.

### Are there any user-facing changes?

Yes this is a new feature.
* GitHub Issue: #41749

Lead-authored-by: Adam Reeve <adreeve@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>