arrow

GH-15199: [C++][Substrait] Allow AGGREGATION_INVOCATION_UNSPECIFIED as valid invocation (#15198)

In our use of our Substrait, we only accept AGGREGATION_INVOCATION_ALL for aggregation invocations. This is fine, but the spec (as per: https://github.com/substrait-io/substrait/blob/main/proto/substrait/algebra.proto#L1221-L1230) automatically reads AGGREGATION_INVOCATION_UNSPECIFIED as AGGREGATION_INVOCATION_ALL, while we reject it. 

This PR accepts AGGREGATION_INVOCATION_UNSPECIFIED past our error checking. Any requested AGGREGATION_INVOCATION_UNSPECIFIED is thus processed as AGGREGATION_INVOCATION_ALL, with no further functionality changes. The basic aggregation test is modified to test this. 
* Closes: #15199

Authored-by: kaesuarez <kaesuarez1423@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>