Implement Chat Server and Asynchronous Client for Simple Chat Application

[nihalpasham]

This pull request introduces a chat server and an asynchronous client implementation for the "simple-chat" challenge. The key features are:

Server:

• User Management: Each user is uniquely identified by a username, ensuring uniqueness at connection.
• Message Broadcasting: Messages sent by a user are broadcasted to all other connected users.
• Disconnect Handling: Users can leave the chat using the leave command, removing them from the active user list.
• Threading for Concurrency: Each client connection is handled in a separate thread, allowing for low-latency communication among multiple users.
• Memory Efficiency: Utilizes Arc<Mutex<>> and Arc<String> for shared state management among threads, minimizing memory overhead.

Client:

• Command-line and Environment Variable Support: Leverages the clap crate to accept host, port, and username as command-line arguments, with environment variables as fallbacks.
• Asynchronous Networking: Employs TcpStream for server connections and mio for non-blocking I/O, ensuring a responsive client experience.
• Event Handling: Listens for incoming messages from the server and user inputs, facilitating real-time interaction.
• Interactive Messaging: Users can send messages with send <MSG> and disconnect with the leave command.

Usage:

To run the server, execute

cd chat-server
cargo run

To run the client, execute:

cd async-chat-client
cargo run -- --host <HOST> --port <PORT> --username "<USERNAME>"