Support HTML-based outstream video

[bretg]

The Problem

As covered in #947, Prebid SDK handles outstream ads different from Prebid.js, which causes a problem in a multiformat banner+video scenario: the ad server line items cannot be set up to make this scenario work properly as currently documented.

There was a proposal to fix the situation with yet-another-key-value-pair, but the alternate proposal is to enhance Prebid SDK to support HTML-based outstream, which makes the problem go away by making the rendering of outstream the same in Mobile as it is in JS.

Proposed Solution

The general idea is to make outstream rendering work in Prebid SDK similar to the way it works in Prebid.js.

There's already a discussion taking place in prebid/Prebid.js#10433 (comment) on how to define the rendererUrl returned from Prebid Server - am thinking this should be applied to Prebid SDK as well as JS.

Need someone to propose how the app developer can define the outstream rendering preferences with the Prebid SDK:

1. GMA SDK render (for backwards compatibility)
2. render with custom SDK
3. render with webview, preferring app-specified renderer
4. render with webview, preferring bidder-specified renderer