Fix nil pointer in Relay conn address
Meanwhile, we create a relayed net.Conn struct instance, it is possible to set the relayedURL to nil.
panic: value method github.com/netbirdio/netbird/relay/client.RelayAddr.String called using nil *RelayAddr pointer
Fix relayed URL variable protection
Protect the channel closing
- Clients now subscribe to peer status changes.
- The server manages and maintains these subscriptions.
- Replaced raw string peer IDs with a custom peer ID type for better type safety and clarity.
