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/common/stats_manager.rs

@@ -664,7 +664,7 @@ impl Default for StatsManager {
 mod tests {
     use super::*;
     use crate::common::stats_manager::{LabelSet, LabelType, MetricName, StatsManager};
-    use crate::proto::cli::{
+    use crate::proto::api::instance::{
         GetPrometheusStatsRequest, GetPrometheusStatsResponse, GetStatsRequest, GetStatsResponse,
     };
     use std::collections::BTreeMap;
@@ -818,16 +818,19 @@ mod tests {
     #[tokio::test]
     async fn test_stats_rpc_data_structures() {
         // Test GetStatsRequest
-        let request = GetStatsRequest {};
-        assert_eq!(request, GetStatsRequest {});
+        let request = GetStatsRequest { instance: None };
+        assert_eq!(request, GetStatsRequest { instance: None });
 
         // Test GetStatsResponse
         let response = GetStatsResponse { metrics: vec![] };
         assert!(response.metrics.is_empty());
 
         // Test GetPrometheusStatsRequest
-        let prometheus_request = GetPrometheusStatsRequest {};
-        assert_eq!(prometheus_request, GetPrometheusStatsRequest {});
+        let prometheus_request = GetPrometheusStatsRequest { instance: None };
+        assert_eq!(
+            prometheus_request,
+            GetPrometheusStatsRequest { instance: None }
+        );
 
         // Test GetPrometheusStatsResponse
         let prometheus_response = GetPrometheusStatsResponse {
@@ -867,7 +870,7 @@ mod tests {
             }
 
             // This simulates what the RPC service would do
-            let _metric_snapshot = crate::proto::cli::MetricSnapshot {
+            let _metric_snapshot = crate::proto::api::instance::MetricSnapshot {
                 name: metric.name.to_string(),
                 value: metric.value,
                 labels,