Making RegistryClient an extension point

Currently NiFi is capable to connect only to the NiFi Registry as a "registry repository". It would be beneficial to give NiFi the capability to depend on other services.

In order to this, I suggest to decouple the registry behaviour from the NiFi Registry as much as it is possible (inlcuding but not only the API and the Resources) and move the actual implementation behind this new API.

To be able to add new implementations, this new API must be an extension point applying the usual NiFi instruments for this. Also it is paramunt to keep the continuity with the current usages and make the implementation capable to process the current REST call format.