Adds mysql DropIndex

[belchior]

Builder API

let query = sql::DropIndex::new()
  .drop_index("users_name_idx")
  .as_string();

let expected_query = "DROP INDEX users_name_idx";

assert_eq!(expected_query, query);

DROP INDEX users_name_idx

Reference

• https://dev.mysql.com/doc/refman/8.4/en/drop-index.html