Flag to disable "unsafe" range sync

[eserilev]

Description

While talking with @dapplion about sync, he mentioned that forward sync is currently unsafe when a Lighthouse nodes current head slot is outside the DA window. In this situation, the node will attempt to forward sync and will receive blocks over RPC that are considered available even though the node is unable to make data availability checks for blocks outside the DA window. Since we are not working backwards from trusted data (like we do in backfill sync), this is inherently unsafe.

An easy way to protect users would be to introduce a new flag that by default disables unsafe forward sync. On start up, the lighthouse node can simply check that the current head slot is within the DA window. If it's outside the DA window, the nod will exit and respond with a helpful error message encouraging users to checkpoint sync. Users can still enable the existing "unsafe" syncing by providing this new flag

This change alongside #7347 can make lighthouse by default a bit safer, especially for non-super users.

I'm looking for buy-in from other team members before working on this issue, please feel free to leave comments/suggestions.

[jimmygchen]

Also some more documentation here: #7011

[eserilev]

woops I'll close this as its a duplicate. I added some comments to #7011