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/proto/magic_dns.proto

@@ -2,7 +2,7 @@ syntax = "proto3";
 
 import "google/protobuf/timestamp.proto";
 import "common.proto";
-import "cli.proto";
+import "api_instance.proto";
 
 package magic_dns;
 
@@ -30,7 +30,7 @@ message DnsRecordList {
 
 message UpdateDnsRecordRequest {
     string zone = 1;
-    repeated cli.Route routes = 2;
+    repeated api.instance.Route routes = 2;
 }
 
 message GetDnsRecordResponse {