PTF: Support IGNORE NULLS in LEAD/LAG functions

While collapsing time-series data, it is common to do LEAD with ignore NULLs to find the previous non-null value for the sequence (when no-change is recorded as a NULL).

Here's a Presto PR, which also implements the same - https://github.com/prestodb/presto/pull/6157

Discovered while porting a RedShift query to LLAP, unsuccessfully