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GH-37938: [Swift] Add initial C data interface implementation (#41342)

Continuation for PR: #39091

This add an initial implementation of the C Data interface for swift. During development it was found that null count was not being properly maintained on the arrow buffers and this change is included as well. Also some minor refactoring was done to existing sources to enable this feature.

This has been tested from Swift calling into C to import data but not from Swift to C exporting data. Test is currently ongoing.
* GitHub Issue: #37938

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