Pass a handle to the integrationApi when calling OfferProducer.produceOffers

[johndayrichter]

Background: Sometimes OfferProducers need to store additional data about an offer that is outside of the OPR offer spec, especially mapping data between the internal representation of an offer in an inventory system and the public representation of the offer in OPR. This could be done very easily using the IntegrationApi.storeValue() (and related) methods, but these are not available to offer producers.

We should pass a handle to the integration API to OfferProducer.producerOffers so that offer producers can take advantage of the key/value store if they need it.