Support hive's udtf grammar.

Now Calcite doesn't support the hive form of UDTF, eg:

select udtf(arg1, arg2) as (seg1, seg2, seg3) from table

Instead, it provide a table function. But we found that hive's udtf form is more easier for users to understand.
We are building a stream sql framework with calcite, which allow user to process data stream by write sql. And most users are migrated from using hive to process offline data. So, use hive's udtf is more friendly to them.

Can you add the feature to support hive udtf in calcite? We really need it.