message: op-chain-ops: add in json struct tags to cdm
  Adds json struct tags to the `CrossDomainMessage` type.
  This is useful for reading and writing this type to
  a file. The `Data` field must be updated to a `hexutil.Bytes`
  in a subsequent PR.
diff: @@ -14,12 +14,12 @@ import (
  // version 1 messages have a value and the most significant
  // byte of the nonce is a 1
  type CrossDomainMessage struct {
  - Nonce *big.Int
  - Sender *common.Address
  - Target *common.Address
  - Value *big.Int
  - GasLimit *big.Int
  - Data []byte
  + Nonce *big.Int `json:"nonce"`
  + Sender *common.Address `json:"sender"`
  + Target *common.Address `json:"target"`
  + Value *big.Int `json:"value"`
  + GasLimit *big.Int `json:"gasLimit"`
  + Data []byte `json:"data"`
  }
  // NewCrossDomainMessage creates a CrossDomainMessage.
  