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GH-42124: [Swift] Add methods for loading and validating builder by type (#42195)

### Rationale for this change
Adding convenience method for loading a builder from a type.  This will be used by ArrowEncoder (upcoming PR) to load builders for a class/structs's properties.

### What changes are included in this PR?
Two methods have been added to ArrowArrayBuilders: 
loadBuilder: method for loading a builder for a Type.
isValidBuilderType: method for checking if a Type is supported by a builder.

### Are these changes tested?
Yes

* GitHub Issue: #42124

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