[C++] Add `Grouper::Reset`

[asfimport]

Adding Grouper::Reset will enable it to be reused in segmented streaming. `See this post.

Reporter: Yaron Gvili / @rtpsw

_{Note: This issue was originally created as ARROW-18311. Please see the migration documentation for further details.}

[pitrou]

Issue resolved by pull request 41352
#41352