Support DISTINCT for SetOperator

[unvalley]

I’d like to parse UNION DISTINCT using BigQuery and MySQL dialect.
This is not included in standard.

BigQuery Reference: https://cloud.google.com/bigquery/docs/reference/standard-sql/query-syntax?hl=ja#set_operators
MySQL Reference: https://dev.mysql.com/doc/refman/8.0/en/union.html

I'm not familiar with other dialects, if other dialects allow UNION DISTINCT, please comment.

[unvalley]

And I'm working on this.

below are what I'm doing (if it's not reasonable change, please let me know).

I'm changing all(bool) field in SetOperation to op_option(Option<SetOperatorOption>).

/// src/ast/query.rs
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub enum SetOperatorOption {
    All,
    Distinct,
}

impl fmt::Display for SetOperatorOption {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            SetOperatorOption::All => write!(f, " ALL"),
            SetOperatorOption::Distinct => write!(f, " DISTINCT"),
        }
    }
}

// ~
pub enum SetExpr {
    // ~
    /// UNION/EXCEPT/INTERSECT of two queries
    SetOperation {
        op: SetOperator,
        op_option: Option<SetOperatorOption>,
        left: Box<SetExpr>,
        right: Box<SetExpr>,
    },
    Values(Values),
}