Implement pub/sub functionality

[xXenvy]

Add pub/sub (message broker) functionality to enable publish/subscribe communication model.
This feature will allow clients to subscribe to specific channels/topics and receive messages published to those channels/topics by other clients.

Requirements:

Message broker functionality:

• Implement the ability for clients to subscribe to channels/topics.
• Implement the ability for clients to publish messages to specific channels/topics.

Error handling:

• Handle cases where clients attempt to publish messages to non-existent channels/topics.
• Handle cases where clients attempt to unsubscribe from channels/topics they are not subscribed to.