Safe block hash uses unrealized justification about prysm

[parithosh]

cc @bsamuels453 (found the initial discrepancy), in case you wanna track the progress here :)

[potuz]

For completeness, the consensus spec says that it's a stopgag to use justification, and the engine spec explicitly is lose in regards to what one is allowed to send:

safeBlockHash: DATA, 32 Bytes - the "safe" block hash of the canonical chain under certain synchrony and honesty assumptions. This value MUST be either equal to or an ancestor of headBlockHash

I'll open the CL spec PR when I get home

[michaelsproul]

Linking to latest discussion on Eth R&D to keep everyone in the loop. Current thinking suggests we don't update the spec to require unrealized justification, and either allow clients to continue doing different things, or define new (explicit) justified/confirmed labels in place of "safe".

https://discord.com/channels/595666850260713488/598292067260825641/1184977086214053939

[potuz]

Discussed it in Discord R&D for a while (mostly with @mkhalinin) and it seemed to have no opposition and be a clearly better option. We didn't really advertise it since unrealized justification wasn't public at the time