feat: Add custom operator (apache#868)

* feat: Add custom operator

From apache#863

- It doesn't parse anything — I'm not sure how to parse ` SELECT 'a' REGEXP '^[a-d]';` with `REGEXP` as the operator... (but fine for my narrow purpose)
- If we need tests, where would I add them?

* Update src/ast/operator.rs

---------

Co-authored-by: Andrew Lamb <[email protected]>

Author: max-sixty

src/ast/operator.rs

@@ -85,6 +85,8 @@ pub enum BinaryOperator {
     BitwiseOr,
     BitwiseAnd,
     BitwiseXor,
+    /// Support for custom operators (built by parsers outside this crate)
+    Custom(String),
     PGBitwiseXor,
     PGBitwiseShiftLeft,
     PGBitwiseShiftRight,
@@ -122,6 +124,7 @@ impl fmt::Display for BinaryOperator {
             BinaryOperator::BitwiseOr => f.write_str("|"),
             BinaryOperator::BitwiseAnd => f.write_str("&"),
             BinaryOperator::BitwiseXor => f.write_str("^"),
+            BinaryOperator::Custom(s) => f.write_str(s),
             BinaryOperator::PGBitwiseXor => f.write_str("#"),
             BinaryOperator::PGBitwiseShiftLeft => f.write_str("<<"),
             BinaryOperator::PGBitwiseShiftRight => f.write_str(">>"),