[Request] Re-Use Buffer for serializeJson()

[schlimmchen]

Is your feature request related to a problem? Please describe.

Currently, when using serializeJson() in the context of the Web API (to write the JSON into a websocket), we allocate a String, serialize the JSON into that String instance, and then call textAll(String) on the websocket. This creates a copy of the buffer inside AsyncWebsocket.

Describe the solution you'd like

Create an AsyncWebSocketSharedBuffer (std::shared_ptr<std::vector<uint8_t>>) rather than a String and have ArduinoJSON serialize the JSON into that buffer using a custom writer. That will merely create a copy of the std::shared_ptr, but not of the buffer itself.

Describe alternatives you've considered

No response

Additional context

No response