feat: introduce `KAFKA_URL` env variable

Targets: #762 (closed)

This MR introduces KAFKA_URL that will be used whenever a Kafka client is instantiated.

How have I tested this

I'm not sure how to test this from the proper workflow so I've just verified it by putting console.log (server/services/kafkaService/kafka.js) and then observed that the URL is printing

Deployment guidelines

1. KAFKA_URL=https://test.ncbi.nlm.nih.gov/books/kafka