feat(config): allow multi word env var keys

[lfleischmann]

Description

Fixes #126

Implementation

• Use envconfig to load environment variables instead of koanf
  • Use split_words struct tag to correctly load vars where config keys consist of multiple words

Tests

Added a (very) small unit test to see whether a config option is correctly overwritten by what is specified through the env var and that multivalued env vars are read correctly. To test for yourself, join config keys by _
(underscore), uppercase the keys, i.e. for server.public.cors.allow_methods
use:

export SERVER_PUBLIC_CORS_ALLOW_METHODS="GET,PUT,POST,DELETE"

Start the backend and observe whether your changes are correct.

[like-a-bause]

fixes #126