Add client configuration validation

We have a config-schema.json already. It would be fine to enforce the configuration to adhere to the schema. Supplying configuration as json file to the app would also be better than a plain JS object, but both options should be kept available.

Loading json and dynamic variables in https://github.com/eoxc/mundi/tree/support-json-config

Validation for example using https://github.com/mozilla/node-convict

https://itnext.io/use-json-schema-for-your-app-config-3c1f4773560f