Receive Arbitrary ORTB Response Fields

[bretg]

Is your feature request related to a problem? Please describe.

Issue https://github.com/prebid/prebid-mobile-ios/issues/926 extends the SDK to get a specific field, the win event url.

There are other fields that might be useful in the future. Specifically, the fields in seatbid.bid.ext.prebid.meta are potentially of future interest.

Describe the solution you'd like

Define a way to obtain additional Prebid Server ORTB response fields without having to upgrade the SDK.

Describe alternatives you've considered

We could just add the known meta fields one-by-one as known response attributes. This would be ok, but the ideal would be able to support to new fields without SDK upgrades.