ActionButton
A notification action button defined by the server in the pw_buttons payload field.
Unlike the legacy Action contract, an action button is cross-platform and its tap is handled by the SDK itself: the notification is dismissed, a pushStat open event carrying the button id is sent, the action is performed and onMessageOpened is called with a message whose getTappedButtonId returns this button's id.
Buttons are read through getButtons. A push may carry at most three of them; malformed buttons are skipped without affecting the rest of the push.