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GH-35607: [C++] Support simple Substrait aggregate extensions (#35608)

### Rationale for this change

See #35607.

### What changes are included in this PR?

A simple `SubstraitAggregateToArrow` converter for `urn:arrow:substrait_simple_extension_function` is added.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #35607

Authored-by: Yaron Gvili <rtpsw@hotmail.com>
Signed-off-by: Li Jin <ice.xelloss@gmail.com>