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ARROW-10406: [C++] Unify dictionaries when writing IPC file in a single shot

This is governed by a new option in IpcWriteOptions.
Unification is supported only for certain value types.
For unsupported types, it will always fail even when the dictionaries are equal.

Closes #9348 from pitrou/ARROW-10406-ipc-file-unify-dicts

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>