Multibid scoring context sharing

[corywalker]

We're testing multibid (#1048) support. I'm looking at some traces and if multiple ads need to be scored from an IG, I see one context getting created for each ad. I'm hoping at the least it could make sense to share contexts across ads in the same IG. If we can share across an even broader scope than IG that could also help with efficiency (for example in the same joining-origin, similar to groupByOrigin).

[MattMenke2]

Note that for bids generated by different IGs, we could at best match the groupByOrigin behavior, rather than relying on same joining-origin, for the same reason groupByOrigin is needed in the first place (leaves / joins from other joining origins can leak data across origins), so we'd need same joining origin, and groupByOrigin to be true.

Unclear if there's reason to be concerned about different IG owners as well, but probably safest to require that to be the same as well.

Sharing context when the IG that generated the bid is the same should of course always be safe.

The main complexity in implementing this is that we only copy some data into the Javascript address space on first access, so if you preserve, say, the auctionConfig argument between JS calls, and only access access member fields from the value passed on the first call in the second call, we can't provide the information to JS, since we've already discarded it. We have the same issue for bidders, where we actually provide the wrong information (I think we provide the data from the current bid instead, which isn't great). Anyhow, we'd need to hook up something in that case for seller worklet reuse as well.

[JensenPaul]

I suspect this might require seller opt-in as reusing a context could break existing scoring scripts (e.g. they might assume a newly initialized context). I imagine we might need to add an executionMode to auction configs. We could consider a frozen-context mode to accelerate things further.