Add support for keys other than strings

[chuck-alt-delete]

Is your feature request related to a problem? Please describe.
I would like to produce kafka record keys that are not strings (eg ints). I'm using KafkaJS version 2.2.3.

Describe the solution you'd like
A clear and concise description of what you want to happen. Please include links to other clients' solutions and/or KIPs for Kafka features if applicable.

Additional context
Here is a minimally reproducible example:

const { Partitioners } = require('kafkajs');
const { faker } = require('@faker-js/faker');
const kafkaConfig = require('./src/kafka/kafkaConfig');

async function produce(k,v) {
    let kafka = await kafkaConfig();
    const producer = kafka.producer({
        createPartitioner: Partitioners.DefaultPartitioner
    });
    await producer.connect();

    try {
        await producer.send({
            topic: "chuck_test",
            messages: [{
                key: k,
                value: v
            }]
        }); 
    
        console.log(`key:${k}\nvalue:${v}`)
    } catch (error) {
        console.error(error)
    }
    
    await producer.disconnect()
}

let my_key = faker.datatype.number(10);
let my_value = faker.internet.email();

produce(my_key,my_value);
produce("0",my_value);

The first record gets an error while the second is fine.

{"level":"ERROR","timestamp":"2023-03-01T20:26:43.464Z","logger":"kafkajs","message":"[Producer] The \"string\" argument must be of type string or an instance of Buffer or ArrayBuffer. Received type number (5)","retryCount":0,"retryTime":266}
...
key:0
value:Rafael17@hotmail.com

[Nevon]

So the keys in Kafka are always just bytes. The fact that we accept a string is just convenience because there's a default way to encode that string into a buffer (see Buffer.from). But for numbers there are many ways you could choose to serialize them, big endian vs little endian, for example. That's why the input type is string | Buffer | null:

kafkajs/types/index.d.ts

Lines 109 to 110 in ff3b111

	export interface Message {
	key?: Buffer | string | null

If you want to use a number as a key you have to decide for yourself how you want that serialized. For example:

const key = Buffer.alloc(8)
key.writeBigInt64BE(BigInt(9007199254740991))

[chuck-alt-delete]

“Bring your own serializers”. Got it! Thanks!