[server] Add a configuration service

[spoenemann]

We currently fetch a lot of configuration values from env variables. It would be very convenient to add a second way to provide server configuration: put a json config file into the running container. Then we can point to that file through a command line option when starting the server.

We should also extract all references to config values into a service where we use keys in kebab-case. If a config file is given, the keys should be looked up in the json object by converting the key into a hierarchical lookup. As fallback, the keys should be looked up in the env variables by converting them to SCREAMING_SNAKE_CASE.