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ARROW-7741: [C++] Adds parquet write support for nested types

1.  Keeps the old version around (without unit tests) under V1 and
    make this version (V2) a default
2.  Reworks PathInternal slightly to preserve column at a time writing
    with chunked columns.  Address @bkietz comment that the vendored
    visitor implementation for variants supports a return type.
3.  While debugging I found we don't preserve all null counts when
    slicing, so included a small patch and unit test for that.

I'll plan on adding bindings in python for the versioning in a follow-up CR that can also be toggled with a environment variable in case regressions are encountered "in the wild"

Closes #6586 from emkornfield/add_parquet_write

Lead-authored-by: Micah Kornfield <emkornfield@gmail.com>
Co-authored-by: emkornfield <emkornfield@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>