Additional update policy "OnDemand"

[aentinger]

This new update policy allows to trigger the transmission of all properties when a certain function (push) is called. In order to configure a property for this update policy it must be initalized using the following syntax in thing_properties.h:

int seconds;
/* ... */
void initProperties()
{
  ArduinoCloud.addProperty(seconds, Permission::Read).publishOnDemand();

In order to trigger the update the ArduinoCloud.push() needs to be called. All properties are transmitted on the next call of ArduinoCloud.update().

void loop()
{
  /* ... */
  ArduinoCloud.update();
  /* ... */
  ArduinoCloud.push();
}

Successfully tested on PROD with

• MKR WiFi 1010 ✔️
• MKR 1000 ✔️
• ESP8266 ✔️

[aentinger]

Here's your update on demand policy @manchoz and @zmoog 👋

[github-actions]

Memory usage change @5b00945615dc5ff6fb830988c1fc1da2f8f81af4

FQBN	Flash Usage	RAM For Global Variables
arduino:samd:mkr1000	🔺 +16	0
arduino:samd:mkrgsm1400	🔺 +16	0
arduino:samd:mkrnb1500	🔺 +16	0
arduino:samd:mkrwan1300	🔺 +16	0
arduino:samd:mkrwifi1010	🔺 +16	0
arduino:samd:nano_33_iot	🔺 +24	0
esp8266:esp8266:huzzah	🔺 +32	0