[C++] Split Type::UNION into Type::SPARSE_UNION and Type::DENSE_UNION

[asfimport]

Similar to the recent Type::INTERVAL split, having these two array types which have different memory layouts under the same Type::type value makes function dispatch somewhat more complicated. This issue is less critical from INTERVAL so this may not be urgent but seems like a good pre-1.0 change

Reporter: Wes McKinney / @wesm
Assignee: Ben Kietzman / @bkietz

PRs and other links:

• GitHub Pull Request #7378

_{Note: This issue was originally created as ARROW-8866. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
This sounds fine to me (assuming the actual PR does show some benefits, of course :-)).

[asfimport]

Antoine Pitrou / @pitrou:
I suppose this'd better wait for the kernel refactor, btw.

[asfimport]

Ben Kietzman / @bkietz:
Should SparseUnionArray still contain three buffers?

[asfimport]

Antoine Pitrou / @pitrou:
It depends how they're represented in the IPC format.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7378
#7378