[PAAPI] Allow bidders to store auctionConfig for future parallel on-device auctions

[FlorentDancy]

Following https://github.com/prebid/Prebid.js/commit/d7c8547c7f81e734cefbf5666a82ada8301c2a8e, Prebid successfully enabled parallel auctions capabilities through the Prebid adapter.

We (Criteo) are attempting to implement this feature for our bid Adapter but encountered an issue that we would like to discuss with the Prebid community to find a viable solution.

To make the parallel auction work, we need to have the auction configuration hardcoded in the adapter, as demonstrated here for the Optable adapter: https://github.com/prebid/Prebid.js/commit/d7c8547c7f81e734cefbf5666a82ada8301c2a8e#diff-df4dcb0ad724a7e25b13cbbcd4dab838d5c0870727843c1f098335fdece0e282R32-R37

We find this approach limiting for several reasons. If we want to support new fields in our auction configuration or modify existing ones for signals that are not asynchronous, we will need to update the adapter. In the long run, this could become cumbersome, as we would have to request publishers to adopt newer versions of the adapter to accommodate updates to our bidding capabilities in PAAPI auctions.

To address this, we suggest allowing the SSP to provide the auctionConfig through an HTTP call (triggered only when parallel PAAPI is enabled) with a short timeout. This would enable us to implement experiments and changes to the auction configurations without necessitating modifications to our adapter.