Pass bidder core name to BidderRequest hook

[linux019]

This example auction request to "/openrtb2/auction" contains bidders alias.

{ 
  "imp": [
        {
            "ext": {
                "prebid": {
                    "bidder": {
                        "rubicon_s2s": {
                            "accountId": 1234,
                            "siteId": 123,
                            "zoneId": 123
                        }
                    }
                }
            }
        }
    ],
 "ext": {
        "prebid": {
            "aliases": {
                "rubicon_s2s": "rubicon"
            }
        }
    }
}

I wrote a module for BidderRequestHook that needs a real bidder name but PBS passes to the hook a bidder alias but not the bidder core name
The resolving of aliases in the hook is inefficient since it is already done upstream in PBS core