storage: use Pebble-side cockroach key schema implementation

[RaduBerinde]

We currently maintain two implementations for the Cockroach-specific key schema, the production one in CRDB and a test-only one in Pebble. Keeping them in sync is error-prone and we have found a handful of bugs that were in one or not the other.

This issue tracks using the pebble-side implementation. A blocker for this work was around test-only assertions but we have recently enabled Pebble invariants for all crdb test builds (#138251).

Jira issue: CRDB-46490

[RaduBerinde]

We want to do this after the 25.1 branch cut.