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ARROW-6625: [C++][Python] Allow concat_tables to null fill missing columns

Also allows it to promote a column of type Null to a null-filled non-Null type.

There are some open questions:

- Do we merge the metadata in schema or fields or not (and what is the semantic of such a merge)?
- Does the order of columns in the result schema need to be deterministic? Note that with the current implementation, two identical call to Concat() might result in different ordering.
- Do we allow concat tables with duplicate column names? how?
- Do we want to make two separate APIs, one strict, one non-strict? their semantics start differing, and implementations don't overlap much.

I'll add more test cases to cover the corner cases once questions got answered.

Closes #5534 from brills/concat and squashes the following commits:

ea57f7cc8 <Zhuo Peng> another round
db47a99d7 <Zhuo Peng> addressed comments
04e1c9ab6 <Zhuo Peng> fixed the windows build.
310cc518d <Zhuo Peng> Added documentation for the field ordering
ee566b30c <Zhuo Peng> Added tests for field ordering after unification.
5c2d53669 <Zhuo Peng> Handle nullable, and made the order deterministic.
c039624c3 <Zhuo Peng> refactored tests
a457feef9 <Zhuo Peng> use ASSERT_RAISES
ac2f3c4a4 <Zhuo Peng>  Allow concat_tables to null fill missing columns.

Lead-authored-by: Zhuo Peng <brillsp@gmail.com>
Co-authored-by: Zhuo Peng <1835738+brills@users.noreply.github.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>