Feat: Allow enabling preview features through Spring Boot / Quarkus Config

[TomCools]

Is your feature request related to a problem? Please describe.
When trying to add Declarative Shadow Variables to the quickstarts, I was forced to add a solverConfig to enable the preview feature. If we want quick feedback on those features, we should make it easier to enable them.

Describe the solution you'd like
Allow enabling of preview features through Spring Boot / Quarkus config.

Describe alternatives you've considered
We could also not do this and accept that preview features will always need a solverConfig file.
This is also related to the weird situation we have now with the Spring/Quarkus config files, where we support some of the configuration, but not all of it.