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/peers/peer_manager.rs

@@ -36,7 +36,7 @@ use crate::{
         PeerPacketFilter,
     },
     proto::{
-        cli::{
+        api::instance::{
             self, list_global_foreign_network_response::OneForeignNetwork,
             ListGlobalForeignNetworkResponse,
         },
@@ -920,7 +920,7 @@ impl PeerManager {
         }
     }
 
-    pub async fn list_routes(&self) -> Vec<cli::Route> {
+    pub async fn list_routes(&self) -> Vec<instance::Route> {
         self.get_route().list_routes().await
     }
 
@@ -1305,8 +1305,8 @@ impl PeerManager {
         self.foreign_network_client.clone()
     }
 
-    pub async fn get_my_info(&self) -> cli::NodeInfo {
-        cli::NodeInfo {
+    pub async fn get_my_info(&self) -> instance::NodeInfo {
+        instance::NodeInfo {
             peer_id: self.my_peer_id,
             ipv4_addr: self
                 .global_ctx