Function onMessageDisplayed

onMessageDisplayed(input): OnMessageDisplayedOutput
Registers a callback that will be invoked on messageDisplayed events.
Parameters
- input: OnMessageInteractionEventHandler
  The input object that holds the callback handler.
Returns OnMessageDisplayedOutput
- An object that holds a remove method to stop listening to events.
Throws
validation: InAppMessagingValidationErrorCode - Thrown when the provided parameters or library configuration is incorrect, or if In App messaging hasn't been initialized.

Example
```
onMessageDisplayed((message) => {
  // use the message
     console.log(message.id);
});
```