Phase 1 - Define sharding_key and desired_sharding_key rules: Completion of phase 1 unblocks: 1. All newly created tables can start adding sharding_key 2. Teams can start working on backfilling the largest and highest priority tables 3. `desired_sharding_key` can allow us implement a less efficient isolation constraint that unblocks implementing validation despite not having all tables with `sharding_key` 4. Have a dashboard that helps us estimate when we'll finish backfilling all sharding keys