drpcyamux: add yamux multiplexing support for concurrent RPCs

[shubhamdhama]

This package enables multiple concurrent RPCs over a single connection using HashiCorp's yamux multiplexer. It's based on https://gitea.elara.ws/Elara6331/drpc but some parts are rewritten to fix critical concurrency issues and resource leaks.

Key improvements over the original:

• Fixed race conditions on connection state using sync.Once and channels
• Eliminated goroutine leaks in stream cleanup and server shutdown paths
• Proper graceful shutdown with WaitGroups throughout the server stack
• Thread-safe idempotent Close() on both client and server
• Simplified error handling in session Accept loop
• Context-aware shutdown that properly unblocks blocking operations

The package provides two main components:

• Conn: Client-side drpc.Conn implementation with yamux multiplexing
• Server: Server that accepts yamux sessions and handles streams concurrently

[shubhamdhama]

I'll update this once and for all in a separate PR.