rename prysmctl cpt latest?

[kasey]

💎 Issue

The subcommand for computing the latest weak subjectivity checkpoint could be improved. One idea is to call it prysmctl cpt weak-subjectivty.

Background

It was originally a companion command to checkpoint sync, so it lives right next to cpt save (used for downloading state+block ssz files for checkpoint sync). Now that these are 2 distinct concepts, we should give it a clearer differentiating name.