feat(proxy): add modify proxy API endpoint and functionality

Implement proxy modification feature including:
- New modifyFrpcProxy function in config.go
- New ModifyProxyHandler in frpcProxyAct.go
- New API endpoint in router.go
- Updated API documentation in docs/api.md

router.go

@@ -38,6 +38,7 @@ func setupRoutes() {
 	http.HandleFunc("/frpcAct/instanceMgr/getInfo", GetInstanceInfoHandler)
 	http.HandleFunc("/frpcAct/instanceMgr/logs", frpLogger.NewInstanceLogHandler(ValidateTokenFromMap).ServeHTTP)
 	http.HandleFunc("/frpcAct/proxyMgr/create", CreateProxyHandler)
+	http.HandleFunc("/frpcAct/proxyMgr/modify", ModifyProxyHandler)
 	http.HandleFunc("/frpcAct/proxyMgr/delete", DeleteProxyHandler)
 	http.HandleFunc("/frpcAct/proxyMgr/list", ListProxiesHandler)