Include the period as part of the KES signing key

Currently, KES signing keys do not contain the period to which they are evolved. This means there is no way to know in which evolution a key is at. This is particularly problematic while designing the KES agent as we will have more than one process/machine handling key evolutions which might not be in sync. It is therefore important to allow the key handlers to understand at which evolution a particular key is at. To that end we should change from the current KES sk design:

```
data SignKeyKES (SumKES h d) =
SignKeySumKES !(SignKeyKES d)
!Seed
!(VerKeyKES d)
!(VerKeyKES d)
deriving Generic
```

to a different data structure that contains the period. However, we might not want to include the period in such a way that it is redundant for each layer of the key (this is because a `Sum6KES` sign key is formed by a `Sum5KES`, and so on).