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.

Start with CONTRIBUTING.md for Java-track contribution and concept-exercise guidance. Then compare the Rust and Elm role-playing-game exercises linked in the issue to understand the proposed basis. Done means the Java track has a new concept exercise introducing optionals, following the track's contribution conventions.

Written by the indexing model from the issue text.