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GH-36317: [C++] Return a BufferVector from CleanListOffsets (#36316)

### Rationale for this change

`CleanListOffsets` is taking output parameters when it could return multiple buffers in a vector exactly how they are supposed to be used to construct the list-like array.

Besides that, having the logic for setting the validity buffer split between it and the caller is unnecessarily confusing.

### What changes are included in this PR?

 - Addition of a new constructor for `MapArray` that can accept a pre-allocated `BufferVector`
 - Removal of output parameter in `CleanListOffsets` and change of return type

### Are these changes tested?

Yes, by existing tests. A pre-existing `MapArray` ctor delegates to this new ctor, so all tests that exercise the old ctor also exercise the new.
* Closes: #36317

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>