Add lambda callback to SlashButtons

The lambda callback should be added to the `SlashButton` class such as `SlashButton#style(String label, Consumer<ButtonClickEvent> lambda)`.

```java
SlashButton.primary("Refresh", event -> { ... });
```

> **Note:** In order to make the lambda callbacks persistent, a pre-processing would be interesting. We would first create a registry/cache for all the callbacks (map index to callback).