add MessageBroadcaster abstraction with filtering and metadata

[anaslimem]

Summary

Added MessageBroadcaster as a cleaner abstraction for broadcasting RPC messages, replacing the raw StreamSender that was previously passed directly to the IO handler.

Changes

• New MessageBroadcaster - A proper abstraction layer with built-in filtering support
• New MessageFilter - Allows excluding specific methods or directions from broadcasts
• New StreamMessageMetadata - Adds timestamp to each message (time since connection started)
• Method tracking in responses - Responses now include the method name they belong to

Example Usage

// Filter out noisy methods
let filter = MessageFilter::new()
    .exclude_method("internal/debug")
    .exclude_direction(StreamMessageDirection::Incoming);

// When receiving messages, they now have timestamps
while let Ok(msg) = receiver.recv().await {
    println!("Method: {:?}, Latency: {:?}", 
        msg.message.method(), 
        msg.metadata.timestamp
    );
}