[R] Expose null placement option through sort bindings

[asfimport]

ARROW-12063 added a null placement option to the sort kernels and to OrderBySinkNode in the C++ library. Expose this through the R bindings.

Reporter: Ian Cook / @ianmcook

Related issues:

• [C++] Add nulls position option to sort functions (depends upon)

_{Note: This issue was originally created as ARROW-14085. Please see the migration documentation for further details.}

[asfimport]

Ian Cook / @ianmcook:
The behavior before this (and the default behavior after it) is that NaN and NA values come after other values. This is consistent with what dplyr::arrange() does, and dplyr offers no option to change this behavior, so I don't think we need to change our arrange() binding at all.

We do need to add this option to the sort_indices and array_sort_indices bindings. 

[asfimport]

Apache Arrow JIRA Bot:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned per project policy. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[github-actions]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.