fix(types): withdrawls index type is int64 (#1555)

Author: sasha-bitfly

backend/pkg/commons/types/exporter.go

@@ -248,7 +248,7 @@ type WithdrawalsByEpoch struct {
 
 type WithdrawalsNotification struct {
 	Slot           uint64 `json:"slot,omitempty"`
-	Index          uint64 `json:"index"`
+	Index          int64  `json:"index"`
 	ValidatorIndex uint64 `json:"validatorindex"`
 	Address        []byte `json:"address"`
 	Amount         uint64 `json:"amount"`