Adds Mysql syntax to DropTable builder

[belchior]

Builder API

let query = sql::DropTable::new()
  // at least one of methods
  .drop_table("users_name_idx")
  .drop_table_if_exists("users_login_idx")
  .as_string();

let expected_query = "DROP TABLE IF EXISTS users_name_idx, users_login_idx";

assert_eq!(expected_query, query);

Reference

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