Rethink our approach to configurations

[pshirshov]

typesafe-config causes several problems:

1. We have to customize pureconfig heavily
2. typesafe-config and pureconfig are not available outside of JVM

Essentially we have at least 2 options to make our config extension working on SJS:

1. We may use https://github.com/ekrich/sconfig for both JVM and SJS. Pureconfig still would have to be re-implemented
2. We may use native JSON objects on SJS and rely on circe to decode them.

This issue is a continuation of #1290