kv,sql: add new KV endpoint that prepares split points and scatters ranges during RESTORE

The current implementation of RESTORE needs to know about node/store boundaries and issue manual AdminSplit and AdminScatter requests to obtain good performance on the resulting tables.

In a multi-tenant environment (especially CC Serverless where we architecturally do not trust SQL servers), providing routine access to node/store details and especially the KV AdminSplit/AdminScatter constitutes a security risk – a malicious tenant server could use these KV commands to disrupt the storage layer for all tenants.

With the goal of disallowing AdminSplit/AdminScatter in every secondary tenant by default, we need a new KV command that “prepares” the target keyspace prior to the AddSST calls by RESTORE.

The KV Command could take as argument:
- the key spans being restored
- an estimate of the data size
- optionally, expected locality/zcfg of the target data

And would internally issue split points and scatter commands that will spread this data ahead of the RESTORE.

Epic: CRDB-26067

Jira issue: CRDB-26068