Modification made for tests (fetch_node_exporter_metrics and stream_node_export_metrics_to_raw_topic)

Added the implementation of "Kafka_listener", "Initiate_collector_backend" and "generate_kafka_reply"

"KafkaProducerService" and "KafkaProducerServiceImpl" class name changed to "TelemetryBackendService" and "TelemetryBackendServiceImpl" respectivily

folder name changed from "telemetry_frontend" to "frontend",  and "client", "service" and "tests" move inside the "frontend"