Support Secondary Bidder feature

[bretg]

Prebid.js supports a publisher auction control that defines a given bidder code as a secondary bidder

"This option specifies bidders that the Prebid auction will no longer wait for before determining the auction has completed. This may be helpful if you find there are a number of low performing and/or high timeout bidders..."

The proposal is that PBS would support this option in the account-level config.

Account-Level Config

{
  "auction": {
      "secondarybidders": ["bidderA", "bidderB"]
   }
}

Requirements

1. If a bidder is defined as secondary by the account-level config, PBS should not wait for that bidder to respond. i.e. when the last primary bidder responds, the auction is over and any secondary bidder that hasn't returned is considered timed out.
2. Any secondary bidder cut short in this way should receive a debug warning in the ORTB response: "secondary bidder timed out, auction proceeded". They receive the same seatnonbid timeout status code 101.
3. If every bidder in the auction is flagged as secondary, then the feature is ignored, and all bidders are considered 'primary'.
4. Aliases are treated separately. i.e. just because a biddercode is defined as secondary does not mean any aliases or root biddercodes are also secondary.

[bretg]

There's a proposal that since we're going to have per-bidder tmax adjustments with #3965, perhaps this feature is not necessary. Community feedback welcome. Lowering priority.