Feat: Add test deserialize helper

[fhennig]

    /// Helper function to deserialize objects with serde. We need this 'singleton_map_recursive' thing, otherwise
    /// untagged enums will not deserialize correctly.
    fn deserialize<'de, T: stackable_operator::k8s_openapi::serde::Deserialize<'de>>(
        input: &'de str,
    ) -> T {
        let deserializer = serde_yaml::Deserializer::from_str(input);
        serde_yaml::with::singleton_map_recursive::deserialize(deserializer).unwrap()
    }

usage:

let auth_class: AuthenticationClass = deserialize(<your string here>);