Create a builder for PipelineDataFlowModel and non-empty consistency between different types

[dlvenable]

Create a PipelineDataFlowModel.Builder class. This will allow us to remove a constructor and have only one private constructor to be used by the builder and by Jackson for JSON deserialization.

Relatedly, the PipelineDataFlowModel should use non-empty serialization for processors. It should also return an empty List<PluginModel> instead of null to provide consistency for clients.