Provide a way to use `std::collections::HashSet` instead of `hashbrown`

[Stupremee]

Currently, all types that use a HashMap or HashSet, use the ones from the hashbrown crate, because we want to support no_std,
and therefore have to use these types.

Unfortunately, that leads to the confusion, that whenever the user gets a HashSet from some API (like JsonWebKey::key_operations), he can't compare or use it in places where a std::collections::HashSet is expected.

We should add some kind of feature way that will use std::collections::HashSet instead of hashbrown.

Maybe add a feature use-hashbrown / hashbrown that will explicitly use hashbrown instead of std. This feature must then be explicitly enabled when the std feature is disabled.