Preserve ORTB seat

[bretg]

We discussed in committee preserving the OpenRTB seat field, which is overridden to 'biddercode' in Prebid.

The initial idea is to store the bidder's original seat in seatbid[].ext.prebid.origseat if it's not the same as seat.

However, this needs to be considered in light of the multiple biddercode feature defined in #2174

[bretg]

Need to understand where the biddercode seat is set: does PBS-core set this value or is it set by the adapters themselves?

[Net-burst]

PBS-Java does this in Core. The assignment of biddercode happens here: https://github.com/prebid/prebid-server-java/blob/4ab3caea2316a5ff04d0961be9a4e3c4896d517b/src/main/java/org/prebid/server/auction/BidResponseCreator.java#L1185

The only real issue is that right now adapters only return extracted bids from $.seatbid[].bid[] and not entire $.seatbid[] object.

[bretg]

Adapters cannot pass seat at all. Rather than changing this interface, there's a proposal that adapters could place the original seat in the metadata object - there agency and advertiser fields there. Or we could add a "seat" field in the meta.

[bretg]

Discussed options:

1. make new meta.Seat
2. adapters can just copy their original seat to meta.AgencyName
3. adapters can just copy their original seat to meta.NetworkName
4. Do nothing. Forget about it - it's not yet a pain point.

Approved option 1.