Construct ML-recommend JSON envelope when using PutMarkLogicRecord

[rjrudin]

As an ML/NiFi user I want to TITLE So that my ingest flow into ML can be as fast as possible by not having to use a REST transform to construct an envelope, and I can also involve a larger set of developers in constructing/maintaining the envelope (the thought being that there would be more developers comfortable with tweaking NiFi processor config vs tweaking a DHF input flow)

• Initial thought is that we can subclass JsonRecordSetWriter and WriteJsonResult to generate a JSON envelope; so this would be a new controller service that extends JsonRecordSetWriter
• The controller service can have properties on it for things that are typically added to the headers in an envelope (i.e. current date/time, DMSDK batch number, FlowFile info, source system name)
• Controller service would also support dynamic properties for stuff to add to the header
• Could go as far as letting the user define the full JSON with references to controller service properties, the instance JSON, etc (I'm thinking of something like using a Freemarker or Velocity template; this would be the escape hatch for giving the user full control over the JSON)

[rjrudin]

Ignoring for now, will reopen in marklogic/nifi if desired.