Restored the derive for the Copy trait

`Copy` trait derive was previously removed as a variant `Custom` with `Ident` data type was added to the enum. This variant has afterwards been removed as deemed, while necessary to fully capture custom indices, extremely rare (the Postgres documentation itself says that creating custom indices is a rather hard task) and it would be very hard to fully capture in a good manner all possible custom variants.

Author: LucaCappelletti94

src/ast/ddl.rs

@@ -1030,7 +1030,7 @@ impl fmt::Display for KeyOrIndexDisplay {
 /// [1]: https://dev.mysql.com/doc/refman/8.0/en/create-table.html
 /// [2]: https://dev.mysql.com/doc/refman/8.0/en/create-index.html
 /// [3]: https://www.postgresql.org/docs/14/sql-createindex.html
-#[derive(Debug, Clone, PartialEq, PartialOrd, Eq, Ord, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Hash)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 #[cfg_attr(feature = "visitor", derive(Visit, VisitMut))]
 pub enum IndexType {