RTBKIT-575: HttpBidderInterface handling of rtbkit specific information (augmentation data)

I didn't want to call it "augmentAuction" because I did not want to conflict with the concept of an "augmentor". This function actually has nothing to do with augmentation, but rather might be used to add or remove any information to the auction just before it is injected into the router.

I needed it because I had to reconstitute the augmentation data in the Auction object from the data that were in the bidrequest's ext field in the rtbkit exchange connector. But the thing is that you can have any logic that you want here. The post-conditions are that you don't "corrupt" the Auction object (i.e null-out some of the fields, put invalid data ...).

However, if you can come up with a better name, I'm open to suggestions.