MysqlPool
This interface is the subset of mysql2 driver's Pool class that
kysely needs.
We don't use the type from mysql2 here to not have a dependency to it.
https://github.com/sidorares/node-mysql2#using-connection-pools
interface MysqlPool {
end(callback: (error: unknown) => void): void;
getConnection(callback: (error: unknown, connection: MysqlPoolConnection) => void): void;
}