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GH-42245: [Swift] Ensure map behavior is the same for all key types (#42246)

### Rationale for this change
Behavior of decoding a map currently changes based on the key type (String or not String)

### What changes are included in this PR?
Added method for handling map to ensure that all maps are decoded the same way.

### Are these changes tested?
Yes

* GitHub Issue: #42245

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