storage,kv: quantify store initialization time

We recently made improvements in store initialization in https://github.com/cockroachdb/cockroach/pull/123959 (https://github.com/cockroachdb/cockroach/issues/109740).

There still may remain areas to optimize above the storage engine. As a first step, we should quantify this startup time for larger stores / multiple stores, via a roachtest. We can then use this harness as a means of identifying where additional improvements could be made, and ensuring that we are not regressing.

Jira issue: CRDB-41957

Epic CRDB-41111