Enhance data size estimation for fields computed by UDFs

Right now if we have columnstat on a column ; we use that to estimate things about the column; - however if an UDF is executed on a column ; the resulting column is treated as unknown thing and defaults are assumed.

An improvement could be to give wide estimation(s) in case of frequently used udf.

For example; consider substr(c,1,1) ; no matter what the input; the output is at most a 1 long string