Add p2p_node_id configuration setting and make node-id argument optional with fallback chain

- Add p2p_node_id field to ChainSettings with empty string default
- Add p2p_node_id to .env.example configuration file
- Change --node-id argument from required to optional with default empty string
- Add node_id resolution with fallback chain: args.node_id -> settings.p2p_node_id -> settings.proposer_id
- Add validation to raise ValueError if no node_id can be resolved
- Update systemd service to use ${

apps/blockchain-node/.env.example

@@ -10,6 +10,7 @@ rpc_bind_port=8006
 # Network
 p2p_bind_host=0.0.0.0
 p2p_bind_port=8001
+p2p_node_id=aitbc1-node
 
 proposer_id=aitbc1-proposer