message: op-chain-ops: withdrawal tx helper
  Adds a method to the `Withdrawal` that turns it into
  a `bindings.TypesWithdrawalTransaction` that is suitable
  for sending via an `ethclient` when sending a transaction.
  This will help to deduplicate code.
diff: @@ -4,6 +4,7 @@ import (
  "errors"
  "math/big"
  + "github.com/ethereum-optimism/optimism/op-bindings/bindings"
  "github.com/ethereum/go-ethereum/accounts/abi"
  "github.com/ethereum/go-ethereum/common"
  "github.com/ethereum/go-ethereum/crypto"
  @@ -139,3 +140,16 @@ func (w *Withdrawal) StorageSlot() (common.Hash, error) {
  slot := crypto.Keccak256(preimage)
  return common.BytesToHash(slot), nil
  }
  +
  +// WithdrawalTransaction will convert the Withdrawal to a type
  +// suitable for sending a transaction.
  +func (w *Withdrawal) WithdrawalTransaction() bindings.TypesWithdrawalTransaction {
  + return bindings.TypesWithdrawalTransaction{
  + Nonce: w.Nonce,
  + Sender: *w.Sender,
  + Target: *w.Target,
  + Value: w.Value,
  + GasLimit: w.GasLimit,
  + Data: w.Data,
  + }
  +}
  