Add Pair iterator sink to IndexedTables

The IndexedTable sink should accept iterator of Pairs of NamedTuples, and then use the columns from the key as index columns and the columns from the value as data columns. This would enable e.g. this query:

@from i in df begin
@select {i.a, i.b} => {i.c, i.d}
@collect IndexedTable
end

Here the index columns would be a and b, and the data columns c and d.

Start by locating the IndexedTable sink and the code handling collected iterator results. Use the provided Julia query as the behavior example: Pair keys should become index columns and Pair values should become data columns, with NamedTuple field names preserved. Add coverage for this query and verify the resulting table has index columns a and b and data columns c and d.