Polymorphic Attribute Support

[steve-r-west]

Some attributes are policymorphic, lets support this so that we can have a cleaner API.

For example rule promotions, depending on the action or strategy, other parameters can be conditional.

Maybe a way of doing this would be

  rule_set.rules.operator: 
      type: CONDITIONAL
      conditions:
      - type: ENUM:gte,gt,lt,le
        when: rule_set.rules.operator.strategy == 'item_sku'

This was split up from #562

Doing the above might be the least breaking behaviour, because the attributes now are a map, and many of the conditions are over loaded, so we couldn't just specify rule_set.rules.operator twice.