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GH-38602: [R] Add missing `prod` for summarize (#38601)



### Rationale for this change

`prod` is currently missing for use in summarize.

### What changes are included in this PR?

Added `prod` for summarize aggregation.

### Are these changes tested?

Yes, included the same tests used for the other aggregation functions for summarize.

### Are there any user-facing changes?

Yes, added `prod` function.

* Closes: #38602

Authored-by: Maximilian Muecke <muecke.maximilian@gmail.com>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>