Add support for `DELTA_FIRST_SLICE`

[akhi3030]

See qkniep/alpenglow#122 for more information.

When producing the first slice of the first slot, the leader should restrict itself to this timeout and when producing the TimeoutCrashedLeader, validators should consider this.

[qkniep]

I just remembered, I think we discussed this somewhere already. The thing is, to enforce this it would have to be a protocol parameter, i.e. Firedancer needs to implement it, maybe we even need a SIMD. If we want to play it safe and allow all behavior that is currently allowed, we have to completely remove the crashed leader timeout...

[akhi3030]

Yeah I remember that discussion vaguely. I suppose, we need to come to an agreement on which path to take and we should probably keep https://github.com/qkniep/alpenglow and this agave in sync with the decision.