Inconsistency in dynamic stored response impid

[bretg]

Turns out issue https://github.com/prebid/prebid-server/issues/2210 wound up with different implementations between PBS-Go and PBS-Java.

PBS-Go uses the spec'd replaceimpid argument to storedbidresponse, but PBS-Java utilizes a ##PBSIMPID## macro in the body of the storedbidresponse.

As a low priority, we should discuss how to resolve this inconsistency.

The reason that the PBS-Java team decided to go with ##PBSIMPID## is that the format of a storedbidresponse could be totally non-ORTB. This means that the storedbidresponse code can't always know where in the body of the storedbidresponse to replace the impid. i.e. it's not always seatbid.bid.impid. Using the ##PBSIMPID## means that the body of the storedbidresponse can be aligned with the incoming request and then given to the bid adapter for processing as usual.

[bretg]

Discussed in backlog grooming and added details to the rationale for the macro. Next step is to discuss how PBS-Go handles this scenario and whether either platform should consider a change.

[bretg]

Discussed in committee. We're asking @VeronikaSolovei9 to take a look as a low priority and document how PBS-Go handles this scenario. Then we can decide a path forward.