Update backend to filter challenges by difficulty

[tschaffter]

Enable to filter challenges by incentive type and difficulty. In preparation of more filters to come, I will follow the approach described in this article that relies on querydsl, filter object and custom repository.

[tschaffter]

Two implementations:

• https://github.com/mumo-dev/spring-query-dsl-sample/blob/main/src/main/java/io/mumo/springquerydsltut/repository/CustomOrderRepositoryImpl.java (code from the article)
• https://gist.github.com/arthurportas/eb3f722c3c8194b52e78

[tschaffter]

Looking for a way to return a Page instead of a List. Maybe with this workaround?

See also from the above thread:

I guess the answer is: don't rely on querydsl-jpa to make your count queries for you, use blaze-persistence-querydsl instead.

See also this example.