Implement Consumer Push API

[kafkiansky]

Closes #28

This PR makes the Push Consumer API available for use, which is also utilized by KV and Object Store to implement key watching functionality.

With proper configuration push consumers enable more reliable processing of individual messages where batch processing is not required. They provide predictable message balancing and allow for scaling by increasing the number of consumers, similar to how consumers and prefetch count work in RabbitMQ.

The Push Consumer API mirrors the Pull Consumer API to ensure a seamless transition between the two models. All you need to do is replace $consumer->pull with $consumer->push:

<?php

declare(strict_types=1);

require_once __DIR__ . '/vendor/autoload.php';

use Thesis\Nats;
use Thesis\Nats\JetStream\Api;
use Thesis\Time\TimeSpan;

$nc = new Nats\Client(Nats\Config::default());
$js = $nc->jetStream();

$stream = $js->createOrUpdateStream(new Api\StreamConfig(
    name: 'EventsStream',
    description: 'Testing Stream',
    subjects: ['events.*'],
));

$consumer = $stream->createOrUpdateConsumer(new Api\ConsumerConfig(
    durableName: 'EventPushConsumer',
    deliverSubject: 'push-consumer-delivery',
    ackPolicy: Api\AckPolicy::Explicit,
    idleHeartbeat: TimeSpan::fromSeconds(5),
    maxAckPending: 1,
));

$subscription = $consumer->push(
    static function (Nats\JetStream\Delivery $delivery): void {
        dump($delivery->message->payload);
        $delivery->ack();
    },
);

$subscription->awaitCompletion();

$nc->drain();