Start implementing LoanBrokerCoverWithdraw.Destination field

- Refactor the IOU payment code out of the Payment transactor and use it
  for different destinations. This should enforce all of the trust line
  rules without having to reinvent a dozen wheels.
- TODO: Same for MPTs.

include/xrpl/protocol/detail/transactions.macro

@@ -802,6 +802,7 @@ TRANSACTION(ttVAULT_WITHDRAW, 69, VaultWithdraw,
     {sfVaultID, soeREQUIRED},
     {sfAmount, soeREQUIRED, soeMPTSupported},
     {sfDestination, soeOPTIONAL},
+    {sfDestinationTag, soeOPTIONAL},
 }))
 
 /** This transaction claws back tokens from a vault. */
@@ -875,6 +876,8 @@ TRANSACTION(ttLOAN_BROKER_COVER_WITHDRAW, 77, LoanBrokerCoverWithdraw,
     noPriv, ({
     {sfLoanBrokerID, soeREQUIRED},
     {sfAmount, soeREQUIRED, soeMPTSupported},
+    {sfDestination, soeOPTIONAL},
+    {sfDestinationTag, soeOPTIONAL},
 }))
 
 /** This transaction creates a Loan */