add safeBlockHash to forkchoiceUpdated

[djrtwo]

add safeBlockHash to engine_forkchoiceUpdated.

note that until a safeBlockHash algorithm for CL is cleanly defined that CL will always submit safeBlockHash = headBlockHash.

Also note that this value is not critical to the consensus of EL but only to be utilized for user facing APIs

[lightclient]

PR looks good - not sure if understand the need for both a finalizedBlockHash and safeBlockHash though.

[djrtwo]

Under normal network conditions, safe is nearly the head if not the head. Safe will not be reorged if certain amount of attestations have been seen, certain synchrony assumption, and honestly assumption. This is useful to potentially display in apis (as opposed to the bleeding edge consensus head)

Finalized is much deeper in chain and has a very strong cryptoeconomic guarantee non reversion

[protolambda]

LGTM