⭐ feat: Migrate from WebSocket to libp2p for peer-to-peer connectivity (#286)

## Description
Whew, some stuff is still not re-implemented, but it's working!

Rabbit's gonna explode with the amount of changes I reckon 😅



<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Introduced a peer-to-peer relay system using libp2p with enhanced
stream forwarding, room state synchronization, and mDNS peer discovery.
- Added decentralized room and participant management, metrics
publishing, and safe, size-limited, concurrent message streaming with
robust framing and callback dispatching.
- Implemented asynchronous, callback-driven message handling over custom
libp2p streams replacing WebSocket signaling.
- **Improvements**
- Migrated signaling and stream protocols from WebSocket to libp2p,
improving reliability and scalability.
- Simplified configuration and environment variables, removing
deprecated flags and adding persistent data support.
- Enhanced logging, error handling, and connection management for better
observability and robustness.
- Refined RTP header extension registration and NAT IP handling for
improved WebRTC performance.
- **Bug Fixes**
- Improved ICE candidate buffering and SDP negotiation in WebRTC
connections.
  - Fixed NAT IP and UDP port range configuration issues.
- **Refactor**
- Modularized codebase, reorganized relay and server logic, and removed
deprecated WebSocket-based components.
- Streamlined message structures, removed obsolete enums and message
types, and simplified SafeMap concurrency.
- Replaced WebSocket signaling with libp2p stream protocols in server
and relay components.
- **Chores**
- Updated and cleaned dependencies across Go, Rust, and JavaScript
packages.
  - Added `.gitignore` for persistent data directory in relay package.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: DatCaptainHorse <DatCaptainHorse@users.noreply.github.com>
Co-authored-by: Philipp Neumann <3daquawolf@gmail.com>

packages/relay/internal/core/mdns.go

@@ -0,0 +1,38 @@
+package core
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+
+	"github.com/libp2p/go-libp2p/core/peer"
+	"github.com/libp2p/go-libp2p/p2p/discovery/mdns"
+)
+
+const (
+	mdnsDiscoveryRendezvous = "/nestri-relay/mdns-discovery/1.0.0" // Shared string for mDNS discovery
+)
+
+type discoveryNotifee struct {
+	relay *Relay
+}
+
+func (d *discoveryNotifee) HandlePeerFound(pi peer.AddrInfo) {
+	if d.relay != nil {
+		if err := d.relay.connectToRelay(context.Background(), &pi); err != nil {
+			slog.Error("failed to connect to discovered relay", "peer", pi.ID, "error", err)
+		}
+	}
+}
+
+func startMDNSDiscovery(relay *Relay) error {
+	d := &discoveryNotifee{
+		relay: relay,
+	}
+
+	service := mdns.NewMdnsService(relay.Host, mdnsDiscoveryRendezvous, d)
+	if err := service.Start(); err != nil {
+		return fmt.Errorf("failed to start mDNS discovery: %w", err)
+	}
+	return nil
+}