producer.produce should be a promise

**Environment Information**
- node-rdkafka version: 3.1.0

**Steps to Reproduce**

```js
const { Producer } = require('node-rdkafka');

async function produce() {
const producer = new Producer({
'client.id': 'client-id',
'metadata.broker.list': 'localhost:9092',
});
await producer.connect();
// ...wait for readiness
producer.produce('local.ssh-sender', null, Buffer.from('Hello Kafka!'), 'key', Date.now());
process.exit(0)
}

produce();
```

**Additional context**

This code will never send the message in the kafka queue because `producer.produce` is not a promise so `process.exit` will be executed (almost) at the same time

Start at the producer.produce API entry point and reproduce the provided Node.js example with a local Kafka broker. Trace how produce reports completion and identify the relevant existing tests or binding code. Done means the example can await produce without exiting before the message is sent.

Written by the indexing model from the issue text.