Add concept: `optionals`

The goal of this issue is to add a new concept to the Java track that introduces `optionals`. The corresponding concept exercise can probably be based on the "Role Playing Game" exercise from the [Rust track](https://github.com/exercism/rust/tree/main/exercises/concept/role-playing-game) or the [Elm track](https://github.com/exercism/elm/tree/main/exercises/concept/role-playing-game).

Instructions on how to contribute to the Java track and how to implement concept exercises can be found in the [contributing guide](https://github.com/exercism/java/blob/main/CONTRIBUTING.md).

As always, leave a comment here if you want to work on this exercise, and we'll assign it to you. In case you have any further questions, feel free to ask here.