GH-44714: [C++] Keep field metadata for keys and values when importing a map type via the C data interface #44715
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Rationale for this change
Import of a map type from the C data interface drops field metadata (including extension type information) which does not happen when importing a map type from IPC or a list of structs. This affects the ability to roundtrip data through pyarrow/Arrow C++ if extension types are not registered.
What changes are included in this PR?
The mechanism to import the map type was changed to align with the method used for IPC import.
Are these changes tested?
Yes.
Are there any user-facing changes?
The current behaviour was surprising/inconsistent, so I think this PR brings it in more line with the current expectation/documentation.