Add multi-format support feature flag

[bretg]

Prebid.js and OpenRTB support "multi-format" ad units: they can have mediatypes with combination of banner, video, and/or native.

There's a different level of support for multi-format amongst bid adapters:

• some fully support it for the mediatypes they do support. i.e. they'll actually return bids of either type
• some will just pick one of the available types and run with that. e.g. if both banner and video are provided, they'll ignore the banner part and consider it a video adunit
• others don't support the arrangement at all, requiring "twin ad units" or even just choking on the request.

It makes sense to have metadata about this feature, but would like to engage the community fo the proper set of values.

Straw suggestion:

multiformat_supported: will-bid-on-any, will-bid-on-one, will-not-bid

Then we can have a note to pubs that any bidder with "will-not-bid" will have to use "twin" adunits in PBJS.

Note that bidders choking on multi-format is a bug IMO. Those bidders need to be fixed to one of the other options: either don't bid or pick one.

Related to prebid/prebid-server#2006 from @muuki88

[muuki88]

• update the "add a bid adapter" docs
• use magnite / rubicon adapter as an example
• fallback to "check with bidders"