Add transform configuration for entities

[malteseduck]

Add @db-serializer and @db-deserializer, with @db-format to allow specification of transforms on entity

[malteseduck]

Create a ServerTransformer interface instead, and add a transformer property on the @Document annotation to specify the class. Then by default when writing or reading this server transformer will generate the appropriate ServerTransform. Could hard-code it in MarkLogicTemplate to call the transform, but maybe someplace better?