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ARROW-11838: [C++] Support IPC reads with shared dictionaries.

The only code change required, AFAICT, is the calculation of num_dicts, which is no longer simply the number of fields, but rather the unique number of id's they point to. I'm calculating this on-demand, as it's quite cheap and not frequently called, but could also (p)re-compute this on every addField.

For now, I've added tests that read materialised data generated from Java, as we don't support writing IPC with shared dictionaries in C++ either yet (and out of scope here).

Down the line, I would like full read & write support for shared dictionaries across at least C++, Python, Java and Julia, so I'll be coming back to this with follow-up PR's where needed. As part of that, I'll also change the tests to no longer rely on materialised files, but use the round-trip mechanism.

Closes #9629 from jmgpeeters/ARROW-11838

Lead-authored-by: jpeeter <jpeeter@loswjpeeter1.intra.gsacapital.com>
Co-authored-by: jmgpeeters <joris.mg.peeters@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>