When set to a non-nil value, will be used as the config-source.
The value must be a source acceptable to clojure.java.io/reader.
Note: Code that sets this must do so prior to loading outpace.config
namespace or any namespace that transitively requires outpace.config.

Returns the first config EDN source found from:
  - explicit-config-source, if non-nil
  - URL to a resource file pointed to by the value of the "resource.config.edn" system property, if present
  - nil if the "resource.config.edn" system property is present, but the resource file is not present
  - the value of the "config.edn" system property, if present
  - "config.edn", if the file exists in the current working directory
otherwise nil.