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/instance_manager.rs

@@ -9,7 +9,8 @@ use crate::{
         scoped_task::ScopedTask,
     },
     launcher::{ConfigSource, NetworkInstance, NetworkInstanceRunningInfo},
-    proto,
+    proto::{self},
+    rpc_service::InstanceRpcService,
 };
 
 pub struct NetworkInstanceManager {
@@ -150,6 +151,26 @@ impl NetworkInstanceManager {
             .map(|instance| instance.value().get_inst_name())
     }
 
+    pub fn filter_network_instance(
+        &self,
+        filter: impl Fn(&uuid::Uuid, &NetworkInstance) -> bool,
+    ) -> Vec<uuid::Uuid> {
+        self.instance_map
+            .iter()
+            .filter(|item| filter(item.key(), item.value()))
+            .map(|item| *item.key())
+            .collect()
+    }
+
+    pub fn get_instance_service(
+        &self,
+        instance_id: &uuid::Uuid,
+    ) -> Option<Arc<dyn InstanceRpcService>> {
+        self.instance_map
+            .get(instance_id)
+            .and_then(|instance| instance.value().get_api_service())
+    }
+
     pub fn set_tun_fd(&self, instance_id: &uuid::Uuid, fd: i32) -> Result<(), anyhow::Error> {
         let mut instance = self
             .instance_map
@@ -340,7 +361,7 @@ fn print_event(instance_id: uuid::Uuid, msg: String) {
     );
 }
 
-fn peer_conn_info_to_string(p: proto::cli::PeerConnInfo) -> String {
+fn peer_conn_info_to_string(p: proto::api::instance::PeerConnInfo) -> String {
     format!(
         "my_peer_id: {}, dst_peer_id: {}, tunnel_info: {:?}",
         p.my_peer_id, p.peer_id, p.tunnel