handle the new grouping behavior using references

[kadinrabo]

We should update the behavior in substrait-java with the spec update 706 changing the definition of grouping sets in AggregateRel to consist of references into a list of grouping expressions instead of consisting of expressions directly.

This change keeps consumers from having to deduplicate the expressions in the grouping sets in order to execute the query or even derive the output schema. Explained in further detail in #700.

[vbarua]

As part of this work, we should also update the Aggregate POJO representation to better match the spec, and make sure that the integration with Calcite in Isthmus works with this new representation.