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GH-43169: [Swift] Add StructArray to ArrowReader (#43335)

### Rationale for this change
Structs have been added for Swift but currently the ArrowReader does not support them.  This PR adds the ArrowReader support

### What changes are included in this PR?
Adding StructArray to ArrowReader

### Are these changes tested?
The next PR for the ArrowWriter will include a test for reading and writing Structs.

* GitHub Issue: #43169

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