For testing purpose, I need to simulate client for generating 100,000 messages per second and send them to kafka topic. Is there any tool or way that can help me generate these
You can make use of Kafka Connect to generate random test data. Check out this custom source Connector https://github.com/xushiyan/kafka-connect-datagen
It allows you to define some settings like message template and randomizable fields to generate test data. Also check out this post for detailed demonstration.