Custom scalar to sql overrides support for DuckDB Unparser dialect(#68)

[sgrebnov]

Which issue does this PR close?

Unparser does not currently support extending dialect with custom handlers for internal UDFs or new engine extensions. Possible workarounds here are creating a custom dialect wrapper around original dialect with overrided scalar_function_to_sql_overrides or using CustomDialect. This is not very convenient and robust as requires tracking changes to the main dialect and reflecting the same improvements in the custom dialect.

Rationale for this change

Propose introducing a pattern and approach for extending scalar_function_to_sql_overrides for dialects with additional handlers, example usage.

/// Create a new instance of the `DuckDB` dialect with support of custom internal UDFs
let dialect = DuckDBDialect::new().with_custom_scalar_overrides(vec![(
    "cosine_distance",
    Box::new(duckdb::cosine_distance_to_sql) as ScalarFnToSqlHandler,
)]);

What changes are included in this PR?

PR introduces new pattern for extending dialects with custom scalar_function_to_sql_overrides handlers starting from DuckDB that could be extended to other dialects next

pub type ScalarFnToSqlHandler = Box<dyn Fn(&Unparser, &[Expr]) -> Result<Option<ast::Expr>> + Send + Sync>;

pub fn with_custom_scalar_overrides(
        mut self,
        handlers: Vec<(&str, ScalarFnToSqlHandler)>,
    ) -> Self {
        ...
        self
    }

Are these changes tested?

Added unit test test_custom_scalar_overrides_duckdb

Are there any user-facing changes?

Breaking change: as we are adding additional field to DuckDB dialect, existing approach to construct DuckDBDialect {} can't be used and needs to be updated to DuckDBDialect::new()