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GH-43092: [Swift] Update ArrowData for Nested Types (allow children) (#43086)

### Rationale for this change
Struct (Nested Types) need children property on ArrowData 

### What changes are included in this PR?
children property added to ArrowData an constructor updated (original constructor moved to convenience constructor so it must call the updated constructor but keeps the original behavior for existing types)

* GitHub Issue: #43092

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