Allow adding custom scalar to sql overrides for DuckDB

[sgrebnov]

Which issue does this PR close?

Allow adding custom scalar to sql overrides for DuckDB, for example

let dialect = DuckDBDialect::new().with_custom_scalar_overrides(vec![
      (
          "cosine_distance",
          Box::new(cosine_distance_to_sql) as ScalarFnToSqlHandler
      ),
  ]);

Concern

1. This is a breaking change for DuckDB as we are adding new field so DuckDBDialect {} must be replaced with DuckDBDialect::new() constructor, I've tried to avoid this by using Default trait but it does not help, with this change we require to update the way DuckDBDialect is created. Note: DuckDB was added by us so most likely only us are affected by this change.

2. I was thinking to add fn get_custom_scalar_overrides()->Option<&HashMap<String, ScalarFnToSqlHandler>> to Dialect Trait as an example of what Dialects should use to provide custom overrides but I don't think we should restrict how Dialects should implement this. We have Type ScalarFnToSqlHandler = Box<dyn Fn(&Unparser, &[Expr]) -> Result<Option<ast::Expr>> + Send + Sync> that specifies the signature and this should be enough for dialects IMO.