[C++] Add a "coalesce" variadic scalar kernel

[asfimport]

Add a variadic scalar compute kernel that works like the SQL coalesce function. It should take arbitrarily many inputs of the same or compatible types and process them row-wise, returning the first non-null value in each row, or returning null if there are no non-null values in the row. 

For example, in the case of 3 integer-type input arrays, this would take inputs:

Array<int32>         Array<int32>         Array<int32>
[                    [                    [
  null,                2,                   3,
  4,                   null,                6,
  null,                null,                9,
  null                 null                 null
]                    ]                    ]

and return output:

Array<int32>
[ 
  2,
  4,
  9,
  null
] 

This should accept scalars and recycle their values.

Reporter: Ian Cook / @ianmcook
Assignee: David Li / @lidavidm

Related issues:

• [C++][Doc] Rename project to make_struct in docs (fixes)
• [C++][Compute] Consolidate fill_null and coalesce (is duplicated by)
• [C++] Improve type support for 'coalesce' kernel (is related to)
• [R] Add binding for coalesce() (is depended upon by)

PRs and other links:

• GitHub Pull Request #10608

_{Note: This issue was originally created as ARROW-13136. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 10608
#10608

[asfimport]

Ian Cook / @ianmcook:
This kernel currently converts dictionaries to strings. Are there any other type limitations to note?

[asfimport]

David Li / @lidavidm:
It'll unpack any dictionary, and it only supports binary-like variable width types. It should have fairly complete support for fixed-width types (except fixed-width list). Let me file a JIRA for complete type support.

[asfimport]

David Li / @lidavidm:
Filed ARROW-13390.

[asfimport]

Ian Cook / @ianmcook:
Thanks!