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GH-34858: [Swift] Initial reader impl (#34842)

- Initial check in for the swift arrow reader impl
- bug fixes found during reader testing
- class/method access modifier changes (mostly from internal to public)

* Closes: #34858

Authored-by: Alva Bandy <abandy@live.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>