core: eth: rpc: implement safe rpc block (#25165)

* core: eth: rpc: implement safe rpc block

* core: fix setHead, panics

rpc/types.go

@@ -61,6 +61,7 @@ type jsonWriter interface {
 type BlockNumber int64
 
 const (
+	SafeBlockNumber      = BlockNumber(-4)
 	FinalizedBlockNumber = BlockNumber(-3)
 	PendingBlockNumber   = BlockNumber(-2)
 	LatestBlockNumber    = BlockNumber(-1)
@@ -92,6 +93,9 @@ func (bn *BlockNumber) UnmarshalJSON(data []byte) error {
 	case "finalized":
 		*bn = FinalizedBlockNumber
 		return nil
+	case "safe":
+		*bn = SafeBlockNumber
+		return nil
 	}
 
 	blckNum, err := hexutil.DecodeUint64(input)
@@ -118,6 +122,8 @@ func (bn BlockNumber) MarshalText() ([]byte, error) {
 		return []byte("pending"), nil
 	case FinalizedBlockNumber:
 		return []byte("finalized"), nil
+	case SafeBlockNumber:
+		return []byte("safe"), nil
 	default:
 		return hexutil.Uint64(bn).MarshalText()
 	}
@@ -168,6 +174,10 @@ func (bnh *BlockNumberOrHash) UnmarshalJSON(data []byte) error {
 		bn := FinalizedBlockNumber
 		bnh.BlockNumber = &bn
 		return nil
+	case "safe":
+		bn := SafeBlockNumber
+		bnh.BlockNumber = &bn
+		return nil
 	default:
 		if len(input) == 66 {
 			hash := common.Hash{}