init rework and config overrides

[rlindner81]

relates to #100

• init now throws if mandatory properties type or fallbackValue are missing or invalid
• init now throws for double invocation and toggles.canInitialize can be used to check if an init is possible
• configuration is read in the order
  • cds model feature toggles (automatically discovered)
  • configuration files, can be multiple
  • runtime configuration in passed inline during initialization, or via cds.env.featureToggles.config in plugin mode
• each of these three "layers" can introduce new feature toggles or override existing feature toggles

TODO

• try out double init blocking with big codebase
• deeper look at override and validation behavior