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Copysets significantly reduce the probability of data loss
in the presence of multi node failures. The RFC for copysets
is presented in https://github.com/cockroachdb/cockroach/pull/32816.

This commit adds interfaces and protos for copysets.
The implementation added later will have two independent tracks:
1. Assignment and persistance of store_id-copyset_id mapping.
2. Rebalancer changes to rebalance replicas into copysets.

Release note: None