Added ping() definition

[izzymg]

The connection interface definition in mysql doesn't actually include ping, but it does exist on both Connection and PoolConnection

const connection = mysql2.createConnection({...});
connection.ping((error) => console.error);

const pool = mysql2.createPool({...});
const conn = await pool.getConnection();
try {
  await conn.ping();
} catch(error) {
  console.error(error);
}

[unional]

Thanks!