Support adding columns with simple casts as default values in ALTER TABLE

[ggnmstr]

Currently, adding a new column with a casted default value fails with a NotImplementedException, even if the underlying value is a simple constant.

ALTER TABLE ducklake.public.default_test ADD COLUMN e varchar DEFAULT 'dv'::varchar ; 
Not implemented Error: We cannot add a column with a non-literal default value. Add the column and then explicitly set the default for new values using "ALTER ... SET DEFAULT"

This happens because 'dv'::varchar has type of ExpressionType::OPERATOR_CAST, not VALUE_CONSTANT:

shared_ptr<DuckLakeFieldData> DuckLakeFieldData::SetDefault(const DuckLakeFieldData &field_data, FieldIndex field_index,
                                                            const ColumnDefinition &new_col, bool add_column) {
	auto result = make_shared_ptr<DuckLakeFieldData>();
	auto new_default =
	    new_col.HasDefaultValue() ? optional_ptr<const ParsedExpression>(new_col.DefaultValue()) : nullptr;
	if (new_default && new_default->type != ExpressionType::VALUE_CONSTANT && add_column) {
		throw NotImplementedException("We cannot add a column with a non-literal default value. Add the column and "
		                              "then explicitly set the default for new values using \"ALTER ... SET DEFAULT\"");
	}
...

This limitation looks inconsistent, as creating a table with a cast as a default value is already supported and works fine:

CREATE TABLE ducklake.public.default_test (a integer, b text DEFAULT 'foo'::text, c integer, d bool DEFAULT true);