feat: Support IGNORE NULLS only for allowed aggregation functions

[comphead]

Currently parser allows to parse queries which IMHO should be failing by unsupported syntax.
Like
SELECT COUNT(*) IGNORE NULLS FROM (values (1), (null), (2));

The allowed list for IGNORE NULLS can be found https://github.com/ronsavage/SQL/blob/master/sql-2016.ebnf

[alamb]

In general I think this crate focuses on parsing the syntax and leaves it to downstream crates (like datafusion) to enforce whatever semantics they want

This is written up a bit here: https://github.com/sqlparser-rs/sqlparser-rs?tab=readme-ov-file#syntax-vs-semantics