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GH-41804: [Swift] Add Struct (Nested) type (#43082)

### Rationale for this change
Nested types are needed in order to implement Struct types.

### What changes are included in this PR?
This change adds the Nested type and a method for type lookup that will be used for building Struct types.

* GitHub Issue: #41804

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