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ARROW-5649: [Integration][C++] Create integration test for extension types

Integration for extension types is only supported on C++.

Also fix the dictionary decoding logic to correctly interpret extension types with a dictionary storage type, and add tests for that.

Closes #6959 from pitrou/ARROW-5649-extension-types-integration

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>