[C++] Implement Schema unification, merging unequal but equivalent schemas

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Some Parquet datasets may contain schemas with mixed REQUIRED/OPTIONAL repetition types. While such schemas aren't strictly equal, we will need to consider them equivalent on the read path

Reporter: Wes McKinney / @wesm
Assignee: Francois Saint-Jacques / @fsaintjacques

PRs and other links:

• GitHub Pull Request #6024

_{Note: This issue was originally created as ARROW-843. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I changed this to a C++ issue since a lot of the datasets logic will be migrated to C++ from where it's in Python now

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Wes McKinney / @wesm:
FYI @bkietz @fsaintjacques @nealrichardson

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 6024
#6024