[SS-96] Setup alternate logic path for upsert_v2

[patrickwwbutler]

This sets up logic to potentially use the new prototype upsert implementation written by @DAlperin and loosely described here: https://www.notion.so/materialize/Upsert-V2-Much-Simpler-Boogaloo-31913f48d37b807fa88bdeafc27c02d9?source=copy_link

The plan is to run this alternate implementation in a shadow cluster that copies the work done in production on certain customer's upsert workloads so we can evaluate for correctness and collect performance metrics before fully switching over.

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