Add Key Rotation to GG18 examples

[HRezaei]

This MR adds Key Rotation support to the GG18 examples. It repeats DKG but keeps the public key fixed. Check README.md for an example on how to run Key Rotation. Once it is done, you can compare the previous key files with the new ones in order to double-check that the public key has remained intact and only local secret shares are refreshed.

[KunPengRen]

How to handle the new party join or party leave key rotation?