Add basic resharding types

[ffuugoo]

Tracked in #4213.

This PR adds a few basic types/variants/fields that are required as the most fundamental building blocks for resharding.

TODO:

• add basic structure with resharding state to Collection
• add second hashring to ShardHolder::rings
• add Resharding replica state

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[ffuugoo]

@timvisee Mostly done, I just need to analyze how ReplicaState is used in a few places to add proper handling for ReplicaState::Resharding. Help welcome. :D

[timvisee]

👍 I think this is good enough for now to hide our stuff from APIs. The OpenAPI spec has not been changed in this PR.

[timvisee]

It implements two things: a new shard replica state and a different hash ring configuration.

This can be done in two separate PRs, and I think we should prefer that. It allow us to discuss better and allows faster merging.