refactor(rpc): Centralize RPC service and unify API (#1427)

This change introduces a major refactoring of the RPC service layer to improve modularity, unify the API, and simplify the overall architecture.

Key changes:
- Replaced per-network-instance RPC services with a single global RPC server, reducing resource usage and simplifying management.
- All clients (CLI, Web UI, etc.) now interact with EasyTier core through a unified RPC entrypoint, enabling consistent authentication and control.
- RPC implementation logic has been moved to `easytier/src/rpc_service/` and organized by functionality (e.g., `instance_manage.rs`, `peer_manage.rs`, `config.rs`) for better maintainability.
- Standardized Protobuf API definitions under `easytier/src/proto/` with an `api_` prefix (e.g., `cli.proto` → `api_instance.proto`) to provide a consistent interface.
- CLI commands now require explicit `--instance-id` or `--instance-name` when multiple network instances are running; the parameter is optional when only one instance exists.

BREAKING CHANGE:  
RPC portal configuration (`rpc_portal` and `rpc_portal_whitelist`) has been removed from per-instance configs and the Web UI. The RPC listen address must now be specified globally via the `--rpc-portal` command-line flag or the `ET_RPC_PORTAL` environment variable, as there is only one RPC service for the entire application.

easytier/src/web_client/mod.rs

@@ -1,6 +1,9 @@
 use std::sync::Arc;
 
-use crate::{common::scoped_task::ScopedTask, tunnel::TunnelConnector};
+use crate::{
+    common::scoped_task::ScopedTask, instance_manager::NetworkInstanceManager,
+    tunnel::TunnelConnector,
+};
 
 pub mod controller;
 pub mod session;
@@ -15,10 +18,12 @@ impl WebClient {
         connector: T,
         token: S,
         hostname: H,
+        manager: Arc<NetworkInstanceManager>,
     ) -> Self {
         let controller = Arc::new(controller::Controller::new(
             token.to_string(),
             hostname.to_string(),
+            manager,
         ));
 
         let controller_clone = controller.clone();