Follow up split of per era CertState

[lehins]

Move into Cardano.Ledger.[Era].State.CertState

We are slowly moving towards all state related functionality defined in Cardano.Ledger.[Era].State sub modules. This needs to be applied to CertState as well.

Create ConwayCertState

Move VState out of ShelleyCertState into the new ConwayCertState

Move VState into cardano-ledger-conway

VState has no business being defined in cardano-ledger-core and needs to migrate into cardano-ledger-conway package

mkCertState

We can't use mkCertState that accepts VState as an argument, because VState will not be available in shelley era.

This isn't really a problem since we can always construct it from an empty CertState, eg:

mkShelleyCertState pState dState = 
  def & certPStateL .~ pState
      & certDStateL .~ dState

Then we can use mkShelleyCertState to define mkConwayCertState

mkCertState pState dState vState = 
  mkShelleyCertState pState dState & certVStateL .~ vState

But those will have to be standalone functions and will not longer be part of the EraCertState type class.

Moreover in ledger rules we can always get away with modifying CertState, instead of constructing a new one.

Originally posted by @lehins in #4861 (comment)