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GH-20385: [C++][Parquet] Reject partial load of an extension type (#33634)

When the parquet reader is performing a partial read it will try and maintain field structure.  So, for example, given a schema of `points: struct<x: int32, y:int32>` and a load of `points.x` it will return `points: struct<x: int32>`.  However, if there is an extension type `points: Point` where `Point` has a storage type `struct<x: int32, y: int32>` then suddenly the reference `points.x` no longer makes sense.
* Closes: #20385

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>