BREAKING CHANGE: various public functions that took the
AggregateFunction.AggregationInvocation proto now take the POJO
equivalent Expression.AggregationInvocation.
AggregateFunction.AggregationInvocation was the only protobuf enum that didn't have a POJO equivalent.
I added one for consistency while I was looking into additional support for window functions.
BREAKING CHANGE: various public functions that took the AggregateFunction.AggregationInvocation proto now take the POJO equivalent Expression.AggregationInvocation.