Implement NiFi service

[evren]

Creation of a Stardog connection should be done by a NiFi ControllerService not the AbstractStardogProcessor.

[PaulJackson123]

The way this works is the consumers of the service (Processors) add a PropertyDescriptor for the service to their SupportedPropertyDescriptors, which causes it to appear as a property in the UI. If the user adds a compatible controller service in scope of the processor, the user can select that service and assign it to that property.

A custom validator could take into account the service and require that either the service or the processor define each required property. I guess if both the service and the processor define the same property then the processor should take precedence (rather than flag the configuration as invalid).

[PaulJackson123]

Conditionally required fields is hard to implement in Nifi.

Validation is only called if a field is set. There's no way to make a field invalid aside from requiring it, in which case it is always invalid if it is not set.

Could come up with a hack where we treat an empty string as a missing value... Meh.