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-web/src/client_manager/mod.rs

@@ -247,9 +247,10 @@ impl ClientManager {
 
 #[cfg(test)]
 mod tests {
-    use std::time::Duration;
+    use std::{sync::Arc, time::Duration};
 
     use easytier::{
+        instance_manager::NetworkInstanceManager,
         tunnel::{
             common::tests::wait_for_condition,
             udp::{UdpTunnelConnector, UdpTunnelListener},
@@ -273,7 +274,12 @@ mod tests {
             .unwrap();
 
         let connector = UdpTunnelConnector::new("udp://127.0.0.1:54333".parse().unwrap());
-        let _c = WebClient::new(connector, "test", "test");
+        let _c = WebClient::new(
+            connector,
+            "test",
+            "test",
+            Arc::new(NetworkInstanceManager::new()),
+        );
 
         wait_for_condition(
             || async { mgr.client_sessions.len() == 1 },