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"

Added parameter type in methods "create_collector_id" and "create_collector_request". Replaced "kpi_id.kpi_id.uuid" with "collector_id.collector_id.uuid" in methods "create_collector_request_a" and "create_collector_request_b".

update methods "export_collector_value" and "write_to_kafka" parameters and return type of "export_collector_value".