This is the first step in a multi-step data migration to allow multiple
DNS writers per TLD. The overall process looks like this:
1. Add a plural DNS writers field with backfill (this commit).
2. Deploy it.
3. Run the ResaveEnvironmentEntitiesCommand to populate this new field
on all entities.
4. Update the code to use the new field everywhere.
5. Deploy it.
6. Delete the now-unreferenced, old deprecated singular value field.
This process is rollback-safe.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161253436