[R] Add bindings for sum and mean compute kernels

[asfimport]

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [R] Provide binding for arrow::compute::CallFunction (is related to)

PRs and other links:

• GitHub Pull Request #7308

_{Note: This issue was originally created as ARROW-6978. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
R should expose arrow::compute::CallFunction so that kernel bindings can be provided without having to touch C++ code

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 7308
#7308