Multi-AdUnit Auction and Winner Selection Logic

[bretg]

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

I learned two things about Prebid SDK that are not aligned with other Prebid products:

1. SDK currently supports doing auctions with only one AdUnit at a time. Doing one auction at a time is poor performance and costs everyone time and money.
2. In "Rendering" mode, SDK selecting a winner without the ability to override. No other part of Prebid makes a winning bid decision that cannot be overridden by publisher code. Use cases include: choosing deals over open market bids and limited bid caching.

Describe the solution you'd like

I would like to propose requirements for a future major version of the SDK to align it with other Prebid products:

1. Support the ability to call for multiple AdUnits at the same time.
2. Support the ability for app developers to provide alternate bid-choosing logic. i.e. don't tie the auction, winner selection, and render together into one inseparable interface.

There are many ways to design these features into the SDK. Will leave that task to the people who know mobile SDKs better.