AggFunctions supported by JdbcAggregate should depend on SqlKind, instead of operator instance

In JdbcAggregate, there is a list of functions that can be implemented and therefore pushed into the underlying database. The list of functions uses operator instance, but it would be better to use SqlKind. This is particularly relevant in case a user creates his own operators (e.g. Drill wraps some of calcite's operators), so comparing instances will not work.