fix(proxyd): Fix compliance with JSON-RPC 2.0 spec by adding optional RPCError.Data (#3683)

* fix: add optional data field to RPCError struct

* fix: formatting lint

* feat(proxyd): add changeset

.changeset/angry-wombats-marry.md

+---
+'@eth-optimism/proxyd': minor
+---
+
+Fixed JSON-RPC 2.0 specification compliance by adding the optional data field on an RPCError

proxyd/rpc.go

@@ -57,6 +57,7 @@ func (r *RPCRes) MarshalJSON() ([]byte, error) {
 type RPCErr struct {
 	Code          int    `json:"code"`
 	Message       string `json:"message"`
+	Data          string `json:"data,omitempty"`
 	HTTPErrorCode int    `json:"-"`
 }
 

proxyd/rpc_test.go

@@ -45,7 +45,7 @@ func TestRPCResJSON(t *testing.T) {
 			`{"jsonrpc":"2.0","result":null,"id":123}`,
 		},
 		{
-			"error result",
+			"error result without data",
 			&RPCRes{
 				JSONRPC: JSONRPCVersion,
 				Error: &RPCErr{
@@ -56,6 +56,19 @@ func TestRPCResJSON(t *testing.T) {
 			},
 			`{"jsonrpc":"2.0","error":{"code":1234,"message":"test err"},"id":123}`,
 		},
+		{
+			"error result with data",
+			&RPCRes{
+				JSONRPC: JSONRPCVersion,
+				Error: &RPCErr{
+					Code:    1234,
+					Message: "test err",
+					Data:    "revert",
+				},
+				ID: []byte("123"),
+			},
+			`{"jsonrpc":"2.0","error":{"code":1234,"message":"test err","data":"revert"},"id":123}`,
+		},
 		{
 			"string ID",
 			&RPCRes{