Support for `IN $placeholder` syntax

[ryanschneider]

Some DBs support using placeholders for IN clauses in prepared statements. For example in DuckDB:

$ duckdb
DuckDB v1.3.0 (Ossivalis) 71c5c07cdd
Enter ".help" for usage hints.
Connected to a transient in-memory database.
Use ".open FILENAME" to reopen on a persistent database.
D PREPARE qry AS SELECT 'a' in $list;
D EXECUTE qry(list := ['a', 'b']);
┌──────────────────────┐
│ contains($list, 'a') │
│       boolean        │
├──────────────────────┤
│ true                 │
└──────────────────────┘

However, currently the parser doesn't handle this, for example:

#[test]
fn test_parse_in_placeholder() {
    let stmt = all_dialects().verified_stmt("SELECT i IN $placeholder");
    dbg!(&stmt);
}

Fails w/ SELECT i IN $placeholder: ParserError("Expected: (, found: $placeholder").

To fix this, I think we would need to make two changes:

First off, the definition of Expr::InList:

    /// `[ NOT ] IN (val1, val2, ...)`
    InList {
        expr: Box<Expr>,
        list: Vec<Expr>,
        negated: bool,
    },

The issue is that list is always a Vec, where in this case we want list to be a Expr::Value w/ value = Placeholder("$placeholder").

If InList supports that, then in parse_in we can do a check like this before the expect_token(LParen):

       if let Token::Placeholder(_) = &self.peek_token_ref().token {
            let placeholder = self.parse_expr()?;
            return Ok(Expr::InList {
                expr: Box::new(expr),
                list: placeholder,
                negated,
            })
        };
        self.expect_token(&Token::LParen)?;

But, how can we cleanly support this, without too much breakage for existing consumers? Note that I considered just putting the placeholder inside the list, but that doesn't work since that would represent IN ($placeholder) which has a very different meaning.