add configuration for using a custom ObjectFactory

this is a first step toward #672 and valkyrie support. we want to be able to
drop in an ObjectFactory implementation that finds and saves using Valkyrie
instead of ActiveFedora.

app/models/concerns/bulkrax/import_behavior.rb

@@ -165,7 +165,7 @@ def active_id_for_authority?(value, field)
     end
 
     def factory
-      @factory ||= Bulkrax::ObjectFactory.new(attributes: self.parsed_metadata,
+      @factory ||= Bulkrax.object_factory.new(attributes: self.parsed_metadata,
                                               source_identifier_value: identifier,
                                               work_identifier: parser.work_identifier,
                                               related_parents_parsed_mapping: parser.related_parents_parsed_mapping,

lib/bulkrax.rb

@@ -16,6 +16,7 @@ class << self
                    :import_path,
                    :multi_value_element_join_on,
                    :multi_value_element_split_on,
+                   :object_factory,
                    :parsers,
                    :qa_controlled_properties,
                    :related_children_field_mapping,

lib/generators/bulkrax/templates/config/initializers/bulkrax.rb

@@ -10,6 +10,9 @@
   # Default is the first returned by Hyrax.config.curation_concerns
   # config.default_work_type = MyWork
 
+  # Factory Class to use when generating and saving objects
+  config.object_factory = Bulkrax::ObjectFactory
+
   # Path to store pending imports
   # config.import_path = 'tmp/imports'