Improve NiFi Registry client dialogs

When adding a new NiFi Registry, the process is broken up into multiple operations - First, only the Name, Type, and Description can be entered/selected.  Only after that's been added, can you then click the pencil icon to actually provide the URL and SSL context properties.

The edit dialog should replace the initial Add dialog.  Also, if only one NiFiRegistryFlowRegistryClient type is available, then it shouldn't be a selectable option.  If more than one type is envisioned in the future, then it should be a property in the Edit dialog.