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ARROW-6772: [C++] Add operator== for interfaces with an Equals() method (#14038)

Addresses [ARROW-6772](https://issues.apache.org/jira/browse/ARROW-6772).

Adds util::EqualityComparable mixin to relevant types, replacing prior operator==() overloads if applicable.

Authored-by: benibus <bpharks@gmx.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>