[DKG] Consider adding an option to set future genesis date

[AnomalRoil]

We might want to add an option to specify a given time for the genesis when creating a new network, so that we can be "compatible" with the existing drand frequency: start the network at the same time the old one would have emitted a round

       Wouldn't it be much nicer to be able to specify a given time for Genesis?

Originally posted by @AnomalRoil in #1237 (comment)