Migrate NiFi Registry into NiFi codebase

When NiFi Registry was created, it was created as a sub-project. There were good reasons for this. The release cadence wasn't yet known, the smaller codebase would make things less intimidating for developers who were newer to the community, etc.

However, we've seen over the last few years that the cons have been much weightier than the pros. There has been a lot of code duplication. Registry often lags in features behind NiFi. There's significantly more maintenance because of this code duplication.

There's also the problem of circular dependencies. NiFi depends on registry, but often a new feature is added to NiFi. We want Registry's data model to support this. So we update registry's data model. But then registry must be released before NiFi can be updated to use the new data model and map into it. 

Also, the VersionedProcessGroup, etc. that exist in the registry data model have become integral to NiFi.

We need to merge these two codebases. This will result in a single release that encompasses both projects. This is much easier for the Release Manager as well as the community who votes on it. The data model will be dramatically easier to update. Registry will benefit from third-party dependency updates to avoid potential CVE's, etc. by improving the maintainability.