message: op-chain-ops: add getter for cdm
  Adds a getter to the `LegacyWithdrawal` that converts
  it into a `CrossDomainMessage`. This simplifies some
  code when doing migrated legacy withdrawals.
diff: @@ -170,3 +170,16 @@ func (w *LegacyWithdrawal) Value() (*big.Int, error) {
  return value, nil
  }
  +
  +// CrossDomainMessage turns the LegacyWithdrawal into
  +// a CrossDomainMessage
  +func (w *LegacyWithdrawal) CrossDomainMessage() *CrossDomainMessage {
  + return &CrossDomainMessage{
  + Nonce: w.Nonce,
  + Sender: w.Sender,
  + Target: w.Target,
  + Value: new(big.Int),
  + GasLimit: new(big.Int),
  + Data: []byte(w.Data),
  + }
  +}
  