UPSTREAM: <carry>: assign shard names to objects

[p0lyn0mial]

This PR applies a shard name to objects.
This is necessary because we don't store the shard name in the objects in the db.
Instead, we need to derive it from the storage key, and then applied after retrieving the object from storage.

Assigning the names is required to:

• enable the watch-cache for the cache server
• instruct the internal kube-controllers how to construct the final URL

[stevekuznetsov]

nit: if we use some struct here to hold our options, I think the rebase cost might be smaller - WDYT?

question: do you need to store the RV of the object on the owning shard here?

[p0lyn0mial]

nit: if we use some struct here to hold our options, I think the rebase cost might be smaller - WDYT?

I can try to open a new PR for it

question: do you need to store the RV of the object on the owning shard here?

not now but in the future, I think it will be needed.

[sttts]

/lgtm
/approve

[stevekuznetsov]

kcp-dev/kcp#342