bug: cannot create table with column with default value of NOW()

[BugenZhao]

In #9609, we removed NOW from the expression proto as it'll never be used by the execution. However, Since we've recently supported non-NULL default values, it's possible that we specify NOW as the default value, then it must be persisted in the catalog.

So we have to introduce NOW in proto again. But the fact that it corresponds to no backend execution function won't change.

risingwave/src/frontend/src/expr/now.rs

Lines 31 to 41 in 771d561

	impl Expr for Now {
	fn return_type(&self) -> DataType {
	DataType::Timestamptz
	}
	fn to_expr_proto(&self) -> ExprNode {
	unreachable!(
	"`Now` should be translated to `Literal` in batch mode or `NowNode` in stream mode"
	)
	}
	}