Subsume `ssDelegations` into `ssActiveStake`

[lehins]

Blocked on #5576

We need to introduce a new type that will contain not only the stake for a staking credential, but also the associated delegation:

data StakeWithDelegation = StakeWithDelegation
  { swdStake :: {-# UNPACK #-} !(NonZero (CompactForm Coin))
  , swdDelegation :: !(KeyHash StakePool)
  }
newtype ActiveStake = ActiveStake
  { unActiveStake :: VMap VB VB (Credential Staking) StakeWithDelegation
  }

This will properly describe the invariant of the active stake in the snapshots: we only care about non-zero stake that is "active", i.e. has an active delegation to a registered stake pool.

This approach will let us remove ssDelegations field and will later reduce the number of tables needed for LedgerHD, since ssActiveStake and ssDelegations can be combined into a single field:

 data SnapShot = SnapShot
-  { ssActiveStake :: !Stake
+  { ssActiveStake :: !ActiveStake
-  , ssDelegations :: !(VMap VB VB (Credential Staking) (KeyHash StakePool)) 

This will also result in some nice computational optimizations:
As part of the resolveInstantStake we already lookup the actual delegation, but ignore the result. We just need to retain the result and this will let us avoid:

• A whole another iteration over accounts map:
  https://github.com/IntersectMBO/cardano-ledger/blob/c0cc1c99232c06d2c65789794f6c10b39cedcdb7/libs/cardano-ledger-core/src/Cardano/Ledger/State/SnapShots.hs#L445C5-L455
• Redundant lookup in the rewards pulser:
  

  cardano-ledger/eras/shelley/impl/src/Cardano/Ledger/Shelley/RewardUpdate.hs

  Line 267 in c0cc1c9

  poolId <- VMap.lookup cred fvDelegs

This work has already been partially started in this branch and can be picked up where it was left off: https://github.com/IntersectMBO/cardano-ledger/tree/lehins/subsume-delegs-to-active-stake

[lehins]

We should also introduce "activeStake" json field for this new representation

Also while changing the decoder we should keeping it backwards compatible and in presence of stake and delegs fields try and decode those, instead of stake+delegs field