Streams 218 - General Purpose Activity Converter Processor

Declares interfaces that modules can implement to:
discover that untyped documents contain specific types
convert those types to Activities

Provides a general ActivityConverterProcessor that handles: 
valid Activities typed as JSON string, ObjectNode, or other POJOs structurally similar to Activity
any specific types with available converters

This PR does NOT modify any existing Classifiers, Serializers, or TypeConverters.  It DOES provide a path for most if not all of them to be deleted and replaced with this more flexible capability.
Is this cast necessary if we've already determined that the document is of type ObjectNode?