fix VE.Direct live data response size

PLLIMIT would not (always) be part of the JSON string transmitted to the
web application and trip it. with the help of the ArduinoJson assistant,
new values to use in the response size calculation were introduced.
also, importantly, a constant offset was added for the DPL status and
the JSON structure sourrounding the MPPTs.

src/WebApi_ws_vedirect_live.cpp

@@ -62,7 +62,8 @@ bool WebApiWsVedirectLiveClass::hasUpdate(size_t idx)
 
 uint16_t WebApiWsVedirectLiveClass::responseSize() const
 {
-    return VictronMppt.controllerAmount() * (1024 + 128);
+    // estimated with ArduinoJson assistant
+    return VictronMppt.controllerAmount() * (1024 + 512) + 128/*DPL status and structure*/;
 }
 
 void WebApiWsVedirectLiveClass::sendDataTaskCb()