Checkpoint sync from non-finalized checkpoint

[michaelsproul]

The Holesky non-finality incident has revealed the utility of being able to sync from a checkpoint that is not finalized.

We've added functionality to consider an arbitrary checkpoint finalized in Lighthouse's database in these PRs:

• Manual finalization endpoint #7059
• Reject attestations to blocks prior to the split #7084

The problem is, the node still has to sync to the desired block before finalizing it.

To improve on this, we could use a similar approach to start from a split that is ahead of finalization. By updating only the split and not other indicators of finalization, we can hopefully avoid the roadblocks which stymied our previous attempt at non-finalized checkpoint sync:

• Allow Checkpoint Sync on State Beyond Finalization #5128