chainntfns: add new option for conf notifications to send block

In this commit, we add a new option for the existing confirmation
notification system that optionally allows the caller to specify that a
block should be included as well.

The only quirk w/ the implementation here is the neutrino backend:
usually we get filtered blocks, we so need to first fetch the block
again so we can deliver the full block to the notifier. On the notifier
end, it'll only be checking for the transactions we care about, to
sending a full block doesn't affect the correctness.

We also extend the `testBatchConfirmationNotification` test to assert
that a block is only included if the caller specifies it.

lntest/mock/chainnotifier.go

@@ -16,8 +16,8 @@ type ChainNotifier struct {
 // RegisterConfirmationsNtfn returns a ConfirmationEvent that contains a channel
 // that the tx confirmation will go over.
 func (c *ChainNotifier) RegisterConfirmationsNtfn(txid *chainhash.Hash,
-	pkScript []byte, numConfs, heightHint uint32) (*chainntnfs.ConfirmationEvent,
-	error) {
+	pkScript []byte, numConfs, heightHint uint32,
+	opts ...chainntnfs.NotifierOption) (*chainntnfs.ConfirmationEvent, error) {
 
 	return &chainntnfs.ConfirmationEvent{
 		Confirmed: c.ConfChan,