frontend: Derive output column names

[xxchan]

In PG

postgres=# select abs(1);
 abs
-----
   1
(1 row)

postgres=# select count(1);
 count
-------
     1
(1 row)

In risingwave

dev=> select abs(1);
 ?column?
----------
        1
(1 row)

dev=> select count(1);
 ?column?
----------
        1
(1 row)

related but seems different: #3690

[xiangjinwu]

To clarify the difference from #3690: the field names mentioned in that issue is internal, while the output column name shown in this issue is part of SQL semantic. For historical context on how they were decoupled and derived independently, check #2465 and #1433

[xiangjinwu]

This affects the result comparison in regress tests (e.g. #5452)

PostgreSQL:
https://github.com/postgres/postgres/blob/86a4dc1e6f29d1992a2afa3fac1a0b0a6e84568c/src/backend/parser/parse_target.c#L1729

RisingWave:

risingwave/src/frontend/src/binder/select.rs

Lines 163 to 180 in 4998ba9

	SelectItem::UnnamedExpr(expr) => {
	let (select_expr, alias) = match &expr.clone() {
	Expr::Identifier(ident) => {
	(self.bind_expr(expr)?, Some(ident.real_value()))
	}
	Expr::CompoundIdentifier(idents) => (
	self.bind_expr(expr)?,
	idents.last().map(|ident| ident.real_value()),
	),
	Expr::FieldIdentifier(field_expr, idents) => (
	self.bind_single_field_column(*field_expr.clone(), idents)?,
	idents.last().map(|ident| ident.real_value()),
	),
	_ => (self.bind_expr(expr)?, None),
	};
	select_list.push(select_expr);
	aliases.push(alias);
	}