Make the `GoldSelector` serializable

**Context & Problem** One core principle of Goldener is the on demand access to pipelines! To enable this for the selection of data, it is required to have a way to load/create back a previously created `GoldSelector`. So far, the only way to store a `GoldSelector` is to pickle it. This is unfortunately not so convenient when the design of objects can still evolve. Let's define the serialization format for the `GoldSelector`! **Proposed Solution** Following the design principle decided in https://github.com/goldener-data/goldener/issues/284: - make `GoldReductionTool` serializable - make `GoldSelectionTool` serializable - make `GoldSelector` serializable