[C++] Add option to handle NAs to VarianceOptions

[asfimport]

When calling the "variance", "stddev", "hash_variance", and "hash_stddev" kernels, there is no option to handle NA values and they are currently ignored - could we have an option added, so that if there are NA values, we get an NA back?

Reporter: Nicola Crane / @thisisnic
Assignee: David Li / @lidavidm

Related issues:

• [R] Use Arrow engine for summarize() by default (blocks)
• [C++] Add options for handling NAs to stddev and variance (is duplicated by)
• [R] Handle na.rm in sd, var bindings (is depended upon by)

PRs and other links:

• GitHub Pull Request #11005

_{Note: This issue was originally created as ARROW-13691. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Presumably we should go all the way and support ScalarAggregateOptions (skip_nulls, min_count) here. (Though consequently I wonder if having min_count default to 1 in C++/Python would lead to surprises.)

[asfimport]

Nicola Crane / @thisisnic:
Supporting both of those options makes a lot of sense!

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 11005
#11005