Refactor debug messages from System.out to SLF4J

Currently, all debug/info/error messages that are contained in the code generated by RagConnect are printed on System.out or System.err. Thus, they cannot be easily controlled.

Idea: Use logging framework API to enable better control, when to log those message, and without enforcing a certain logging framework.

Important: This introduces a fixed, required dependency group: 'org.slf4j', name: 'slf4j-api', version: '1.7.25' for code generated by RagConnect. This has to be documented, and is a breaking change.