op-chain-ops: sent message migration-data parsing

Add go code for reading output of the `migration-data` package.

op-chain-ops/genesis/migration.go

+package genesis
+
+import (
+	"encoding/json"
+	"os"
+
+	"github.com/ethereum-optimism/optimism/op-chain-ops/crossdomain"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+)
+
+// SentMessageJSON represents an entry in the JSON file that is created by
+// the `migration-data` package. Each entry represents a call to the
+// `LegacyMessagePasser`. The `who` should always be the
+// `L2CrossDomainMessenger` and the `msg` should be an abi encoded
+// `relayMessage(address,address,bytes,uint256)`
+type SentMessageJSON struct {
+	Who common.Address `json:"who"`
+	Msg hexutil.Bytes  `json:"msg"`
+}
+
+// NewSentMessageJSON will read a JSON file from disk given a path to the JSON
+// file. The JSON file this function reads from disk is an output from the
+// `migration-data` package.
+func NewSentMessageJSON(path string) ([]*SentMessageJSON, error) {
+	file, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+
+	var j []*SentMessageJSON
+	if err := json.Unmarshal(file, &j); err != nil {
+		return nil, err
+	}
+
+	return j, nil
+}
+
+// ToLegacyWithdrawal will convert a SentMessageJSON to a LegacyWithdrawal
+// struct. This is useful because the LegacyWithdrawal struct has helper
+// functions on it that can compute the withdrawal hash and the storage slot.
+func (s *SentMessageJSON) ToLegacyWithdrawal() (*crossdomain.LegacyWithdrawal, error) {
+	data := make([]byte, 0, len(s.Who)+len(s.Msg))
+	copy(data, s.Msg)
+	copy(data[len(s.Msg):], s.Who[:])
+
+	var w crossdomain.LegacyWithdrawal
+	if err := w.Decode(data); err != nil {
+		return nil, err
+	}
+	return &w, nil
+}