Move `reference.conf` environment variables to cli modules

[mdedetrich]

Currently environment variable overrides are defined in reference.conf in core modules, i.e. https://github.com/aiven/guardian-for-apache-kafka/blob/main/core-backup/src/main/resources/reference.conf#L1-L30 . When thinking about this, ideally they should be moved to the equivalent cli porject as application.conf files since the environment variables are only relevant when you actually run your application.

Furthermore if someone uses Guardian core modules as a library the user of that library may not want to expose those environment variables under these name/s (or even at all). This can also help alleviate #91 since we can encapsulate any such changes in the cli modules and leave the core Guardian modules as clean Scala libraries.

Is this a feature you would work on yourself?

• I plan to open a pull request for this feature