Add delegation field to type WalletStateUpdate.

This represents any stake delegation that may have occurred during the
transition from one wallet state to another wallet state.

lib/core/src/Cardano/Wallet/Primitive/Model.hs

@@ -67,6 +67,7 @@ import Cardano.Wallet.Primitive.Types
     , EpochLength (..)
     , FeePolicy (..)
     , Hash (..)
+    , PoolId (..)
     , SlotLength (..)
     , SlotParameters (SlotParameters)
     , StartTime (..)
@@ -279,6 +280,8 @@ data WalletStateUpdate s = WalletStateUpdate
       -- ^ The updated state of the wallet.
     , transactions :: ![(Tx, TxMeta)]
       -- ^ Transactions that occur during the state transition.
+    , delegation :: !(Maybe PoolId)
+        -- ^ Any stake delegation that occurs during the state transition.
     } deriving (Generic, Show, Eq)
 
 -- | Apply a single block to the wallet. This is the primary way of making a
@@ -292,6 +295,8 @@ applyBlock w@(Wallet !u _ s bp) !b = WalletStateUpdate
     { initialState = w
     , updatedState = Wallet u' (b ^. #header) s' bp
     , transactions = txs
+    , delegation = Nothing
+        -- TODO: Determine whether any stake delegation has occurred.
     }
   where
     ((txs, u'), s') = prefilterBlock b u s