BigQuery's dialect SELECT AS STRUCT and SELECT AS VALUE ends up with Parse Error

[kubaracek]

Currently syntax for BigQuery's SELECT AS STRUCT (documentation) and SELECT AS VALUE (documentation) raises a parsing error.

They should both produce a Value table

Minimal examples for tests:

#[test]
fn parse_select_as_struct_syntax() {
    // syntax: SELECT AS STRUCT 1 a, 2 b
    let sql = r#"SELECT AS STRUCT 1 a, 2 b"#;
    ...
}

#[test]
fn parse_select_as_struct_syntax() {
    // syntax: SELECT AS VALUE STRUCT(123 AS a, FALSE AS b)
    let sql = r#"SELECT AS VALUE STRUCT(123 AS a, FALSE AS b)"#;
    ...
}