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ARROW-1636: [C++][Integration] Implement integration test parsing in C++ for null type, add integration test data generation

This enables integration testing of null type for C++ and Java. I was a bit surprised that the cross-compatibility tests passed so I would appreciate a sanity check from a Java developer to see if there are integration tests covering the integration-JSON-format parsing and null type IPC round trip. If there are not, let's ensure there are relevant JIRA issues so it can be tested and kept working

Closes #6368 from wesm/ARROW-1636 and squashes the following commits:

eb3620bb1 <Antoine Pitrou> Add TODOs
d028dc46d <Wes McKinney> Implement integration test parsing in C++ for null type, add integration test data generation

Lead-authored-by: Wes McKinney <wesm+git@apache.org>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>