refactor(frpc): restructure instance management and config handling

- Move instance-related structs and functions to config.go
- Remove serverAddr, serverPort, and authMethod from database schema
- Implement new config parsing and encoding with nested key support
- Update service management to use instanceID instead of username/name
- Add GetServiceNameByInstanceID helper function
- Update API documentation for auth.method field change

README.md

@@ -66,6 +66,7 @@ For detailed API documentation, please see [docs/api.md](docs/api.md)
 - [ ] Add frpc instance running status management API
 - [ ] Add frpc instance log display API
 - [ ] Fix random database lock when processing logs
+- [ ] Add frpc createdBy storage and display
 
 ## License