Refactor detectors: improve modularity of the OpenNMS GRPC Server

Detector implementations have 2 key responsibilities: (1) "RPC Handling" (marshalling and unmarshalling requests, ...) and (2) actual detection logic (e.g. sending an HTTP request to a web service).

Refactoring the OpenNMS GRPC Server for improved modularity and code readability helps in moving torward clean separation of the Detector responsibilities.