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/rpc_service/stats.rs

@@ -0,0 +1,50 @@
+use std::sync::Arc;
+
+use crate::{
+    instance_manager::NetworkInstanceManager,
+    proto::{
+        api::instance::{
+            GetPrometheusStatsRequest, GetPrometheusStatsResponse, GetStatsRequest,
+            GetStatsResponse, StatsRpc,
+        },
+        rpc_types::controller::BaseController,
+    },
+};
+
+#[derive(Clone)]
+pub struct StatsRpcService {
+    instance_manager: Arc<NetworkInstanceManager>,
+}
+
+impl StatsRpcService {
+    pub fn new(instance_manager: Arc<NetworkInstanceManager>) -> Self {
+        Self { instance_manager }
+    }
+}
+
+#[async_trait::async_trait]
+impl StatsRpc for StatsRpcService {
+    type Controller = BaseController;
+
+    async fn get_stats(
+        &self,
+        ctrl: Self::Controller,
+        req: GetStatsRequest,
+    ) -> crate::proto::rpc_types::error::Result<GetStatsResponse> {
+        super::get_instance_service(&self.instance_manager, &req.instance)?
+            .get_stats_service()
+            .get_stats(ctrl, req)
+            .await
+    }
+
+    async fn get_prometheus_stats(
+        &self,
+        ctrl: Self::Controller,
+        req: GetPrometheusStatsRequest,
+    ) -> crate::proto::rpc_types::error::Result<GetPrometheusStatsResponse> {
+        super::get_instance_service(&self.instance_manager, &req.instance)?
+            .get_stats_service()
+            .get_prometheus_stats(ctrl, req)
+            .await
+    }
+}