op-node: Export BlockToBatch and add ChannelOut.AddBatch

We need to access the batch data directly in the batcher when tracking
the sequencer window. For that reason, export BlockToBatch to be able to
create the batch from a block there and then add it to ChannelOut without
converting twice.

op-node/rollup/derive/channel_out.go

@@ -75,10 +75,27 @@ func (co *ChannelOut) AddBlock(block *types.Block) (uint64, error) {
 	if co.closed {
 		return 0, errors.New("already closed")
 	}
-	batch, err := blockToBatch(block)
+
+	batch, err := BlockToBatch(block)
 	if err != nil {
 		return 0, err
 	}
+	return co.AddBatch(batch)
+}
+
+// AddBatch adds a batch to the channel. It returns the RLP encoded byte size
+// and an error if there is a problem adding the batch. The only sentinel error
+// that it returns is ErrTooManyRLPBytes. If this error is returned, the channel
+// should be closed and a new one should be made.
+//
+// AddBatch should be used together with BlockToBatch if you need to access the
+// BatchData before adding a block to the channel. It isn't possible to access
+// the batch data with AddBlock.
+func (co *ChannelOut) AddBatch(batch *BatchData) (uint64, error) {
+	if co.closed {
+		return 0, errors.New("already closed")
+	}
+
 	// We encode to a temporary buffer to determine the encoded length to
 	// ensure that the total size of all RLP elements is less than or equal to MAX_RLP_BYTES_PER_CHANNEL
 	var buf bytes.Buffer
@@ -164,8 +181,8 @@ func (co *ChannelOut) OutputFrame(w *bytes.Buffer, maxSize uint64) (uint16, erro
 	}
 }
 
-// blockToBatch transforms a block into a batch object that can easily be RLP encoded.
-func blockToBatch(block *types.Block) (*BatchData, error) {
+// BlockToBatch transforms a block into a batch object that can easily be RLP encoded.
+func BlockToBatch(block *types.Block) (*BatchData, error) {
 	opaqueTxs := make([]hexutil.Bytes, 0, len(block.Transactions()))
 	for i, tx := range block.Transactions() {
 		if tx.Type() == types.DepositTxType {