rpc: add BlockNumber.MarshalText (#23324)

Currently rpc.BlockNumber is marshalled to JSON as a numeric value, which is
wrong because BlockNumber.UnmarshalJSON() wants it to either be hex-encoded
or string "earliest"/"latest"/"pending". As a result, the call chain

    rpc.BlockNumberOrHashWithNumber(123) -> json.Marshal() -> json.Unmarshal()

fails with error "cannot unmarshal object into Go value of type string".

rpc/types.go

@@ -98,6 +98,22 @@ func (bn *BlockNumber) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
+// MarshalText implements encoding.TextMarshaler. It marshals:
+// - "latest", "earliest" or "pending" as strings
+// - other numbers as hex
+func (bn BlockNumber) MarshalText() ([]byte, error) {
+	switch bn {
+	case EarliestBlockNumber:
+		return []byte("earliest"), nil
+	case LatestBlockNumber:
+		return []byte("latest"), nil
+	case PendingBlockNumber:
+		return []byte("pending"), nil
+	default:
+		return hexutil.Uint64(bn).MarshalText()
+	}
+}
+
 func (bn BlockNumber) Int64() int64 {
 	return (int64)(bn)
 }