Reimplement rule loading mechanics

[ekneg54]

Currently the mechanics to load rules are implemented here in the processor abc class: https://github.com/fkie-cad/Logprep/blob/main/logprep/abc/processor.py#L246

This Code Fragment should be extracted and rewritten to reuse it in the auto-rule-tester and to mitigate the issue #749 .

Perhaps we could rewrite it as isolated class RuleLoader which is able to load rules from a given directory recursively or from a single file or from a given dict.